Readers' Rating:

Rate this article

Send to a friend

Proteins, through their constituent amino acids, are an important dietary component especially for cats.

It is believed that many of the metabolic adaptations of the cat are the direct result of its evolutionary dietary habits. Specialising in the hunting, catching and ingestion of other animals throughout evolution, cats would have ingested a diet devoid of carbohydrates and containing an excess of protein relative to energy (Morris, 2001).

To cope with the dietary nutrient balance associated with a carnivorous lifestyle, a number of metabolic adaptations were either not developed or when already present became unnecessary. Adaptations relating to the protein component of the diet can today be found in feline metabolism of arginine, taurine, non-essential amino acids and tryptophan.

In addition, the cat has developed a sensitive taste for protein, hydrolysed protein and individual amino acids (Boudreau and White, 1978).

The present contribution will focus on the role of protein in the diet of cats and discuss aspects of dietary formulation important to the nutrition of domestic cats.


The role of protein in palatability

Taste preference means that an animal will select certain types of foods in preference to others and accordingly those foods will be regarded as being more palatable. The importance of palatability is evident, as a diet can be complete and balanced nutrition but if it is not eaten it does not provide nutrients to the animal and therefore is of no value. The palatability of pet foods is influenced by many factors including food texture, food composition, ingredients, smell, taste, temperature, past experience of the animal, heat treatment, etc.

In the oral and nasal cavities of cats there are a variety of chemical sensory systems that are involved in the selection of food. The chemical systems in the nasal cavity are predominantly focussed on volatile compounds while those in the oral cavities are responsive to water soluble compounds (Boudreau and White, 1978). Boudreau and White studied the chemoresponsiveness of neurons in the geniculate ganglion to various pure compounds in cats and dogs.

Three different chemoresponsive neural groups (I, II and III) cover 90% of the total chemoresponsive tongue units in cats while in dogs four groups (A, B, C and D) were identified. Group I units in cats were found to behave similarly to class B units in dogs and responded to malic acid, ATP and sodium phosphate (NaH₂PO₄). Certain amino acids (e.g. L-proline, L-cysteine, L-lysine and L-taurine) were most effective in stimulating group II units while Ltryptophan, L-isoleucine and L-phenylalanine were found to be inhibitors of group II units in cats (Boudreau and White, 1978).

Compounds normally found in vertebrate tissues such as the dipeptides carnosine (ß-alanyl-L-histidine) and anserine (ßalanyl- methyl-L-histidine), and di- and triphosphate nucleotides were shown to be potent stimuli for either group I, II and III units in cats.

Certain compounds such as creatine, creatinine, bases and polyamines were shown to have inhibitory effects on group II neurons. Sugars (glucose, fructose, etc.) do not appear to stimulate taste receptors in the cat (Boudreau and White, 1978). The latter was also found by Beauchamp et al. (1977), who conducted two choice tests in wild cats. No preference was found for solutions of sucrose, fructose, D-glucose, D-galactose or D-mannose over water.

The studies by Beauchamp et al. (1977) also highlight the importance of protein and amino acids in the palatability of foods for cats. Testing protein mixtures (casein, lactalbumin and soy hydrolysates) dissolved in water at four different concentrations (3.0, 1.5, 0.75 and 0.375 % w/v) against water, all three hydrolysates were shown to be preferred over water. For casein and lactalbumin hydrolysates, there were significant concentration effects, with increased concentrations being preferred.

Comparing the hydrolysate of casein and soy at a concentration of 1.5% directly, the casein hydrolysate was significantly preferred by cats.

Beauchamp et al. (1977) also showed that cats were indifferent to the amino acid glycine when dissolved in water. L-glutamic acid, however, was significantly avoided by the cats while in one experiment there was a preference for a solution containing alanine over water. Proline was found to be significantly preferred by wild cats compared to water. The latter is in line with observations made by Boudreau and White (1978) that proline is a potent stimulant to group II units in cats.

It is nowadays common practise to use hydrolysates manufactured using animal tissues as palatants in diets for cats. Extruded dry diets, without the addition of a palatant sprayed onto the outside of the kibble, are bland in taste and relatively unpalatable to cats. The protein and amino acid component of these palatability enhancers is an important component in conveying palatability of the finished product.


Protein metabolism of cats

Protein metabolism is “the history in the body of some 20 amino acids which occur in the proteins of the diet” (Munro, 1964). It therefore includes the digestion and absorption of dietary proteins, anabolic and catabolic processes of amino acids and nitrogenous compounds derived from amino acids (dietary and endogenous) and the excretion/loss from the body of nitrogenous compounds derived from amino acids.

There are many unique aspects of feline protein metabolism that point toward a specialisation of this species of ingesting animal tissues. The more well known adaptations include a lack of niacin synthesis from tryptophan, a low rate of arginine synthesis, a low rate of taurine synthesis from cysteine and a lack in regulating aminotransferases involved in general nitrogen metabolism.

An excellent review of the adaptations that have occurred in the metabolic pathways of tryptophan, arginine and taurine in cats was recently published by Morris (2001). As a result of the numerous metabolic adaptations of cats toward the ingestion of an animal tissue diet, commercial cat foods are formulated such that the final product contains sufficient protein, taurine, niacin, vitamin A, vitamin D and arachidonic acid for optimal growth and health.

There are also less well-known metabolic adaptations in the protein metabolism of cats that provide further evidence of the carnivorous nature of this species and affect formulation of feline diets.

Felinine (2-amino-7-hydroxy-5,5-dimethyl-4- thiaheptanoic acid, Figure 1) is a sulphur containing amino acid present in the urine of certain members of the Felidae family including the domestic cat.

Discovered by Datta and Harris (1951), 24 hr felinine excretion levels of entire male cats have been reported to be 25 mg/kg bodyweight while castrated male, entire female and spayed female cats excrete 8.5, 7.5 and 4.1 mg/kg body weight, respectively (Hendriks et al., 1995).

Precursors of felinine synthesis were recently shown to include cysteine and methionine with cysteine being a more immediate precursor for felinine synthesis than methionine (Hendriks et al., 2001). It has been hypothesized since the discovery of felinine that this amino acid is present in the blood of cats at extremely low levels or synthesized in the kidney and directly excreted in the urine.

Recently however, Rutherfurd et al. (2001) identified a felinine containing tripeptide (γ-glutamylfelinylglycine) in the blood of domestic cats opening the possibility that felinine synthesis occurs in other tissues (most likely the liver) and is transported to the kidney in a tripeptide form.

Synthesis of γ-glutamylfelinylglycine has been hypothesized to occur in the liver through a condensation reaction of an allylic carbonium ion and the amino acid cysteine in glutathionine (Hendriks et al., 2001). γ-Glutamylfelinylglycine once transported to the kidney can yield free felinine, which can be excreted in the urine through the action of the common glutathione degradation enzymes, γ- glutamyltransferase and aminopeptidase M. This latter ‘liberation’ of felinine, however remains to be proven.

Chemical synthesis of the γ- glutamyltransferase was recently achieved (Woolhouse et al., 2002), confirming the structure of this tripeptide in the blood of domestic cats unequivocally. Current estimates of the sulphur amino acid requirements of adult cats have not taken into account the normal excretion of felinine by tomcats.

It can be calculated that for an adult domestic tomcat at maintenance, approximately 30% of the sulphur amino acids would be required for the synthesis of felinine.

Figure 1. Chemical structure of felinine and isovalthine.

Isovalthine (2-amino-5-carboxy-6-methyl-4- thiaheptanoic acid, Figure 1) is another sulphur-containing amino acid excreted by domestic cats and lions (Oomori and Mizuhara, 1960; Greaves, 1965). During the 1960s much research was conducted on isovalthine yielding contradictory and confusing information with regard to its biosynthesis. Unlike felinine, which can only been found in urine of certain Felidae species, isovalthine has been shown to be present in the urine of humans suffering from hypercholesterolemia (Oomori and Mizuhara, 1960).

This amino acid however cannot be found in the urine of healthy humans, dogs or horses but is present in the urine of normal healthy cats (Oomori and Mizuhara, 1960; Hendriks, unpublished). In addition, guinea pigs can also be made to excrete isovalthine and therefore this amino acid seems to be synthesised as part of general mammalian protein metabolism. Attempts to elucidate the biochemical synthesis pathway of isovalthine during the 1960s were largely unsuccessful, with the exception that it was shown that the sulphur atom in isovalthine could originate from either methionine or cysteine, with cysteine being a more direct source (Ubuka et al., 1964).

The origin of the carbon skeleton in isovalthine remains, even after many experiments, undetermined. Although many studies were aimed at identifying the source of the carbon skeleton, contradictory evidence was obtained. Therefore, isovalthine biosynthesis and metabolism remains largely unknown.

It seems however that isovalthine synthesis is part of the general protein metabolism of mammals but that it is synthesised only as part of normal protein metabolism in species belonging to the Felidae family. Further research is required to determine the importance of this amino acid in the protein metabolism of animals and the relationship of this amino acid to cholesterol metabolism. The amount of dietary sulphur amino acids required for normal isovalthine excretion in cats remains unknown due to the lack of information on 24 hr isovalthine excretions in cats.

Another aspect of feline protein metabolism that provides further evidence of metabolic adaptations to a carnivorous diet throughout evolution is the high endogenous gut amino acid losses of cats. Hendriks et al. (1996) determined the ileal endogenous amino acid losses in adult cats fed a protein-free diet and showed that the losses in cats are approximately 2- 3 times those of humans and other animal species such as rats, pigs and chickens (Table 1). The adult dog appears to have slightly lower endogenous amino acid losses at the end of the small intestine compared to the adult cat but higher than the rat, pig, chicken and humans. This is consistent with the classification of the dog as an omnivorous carnivore.

Table 1. Endogenous amino acid loss at the end of the small intestine determined using the enzyme hydrolysed casein method in various animal species1.

1Data from Moughan (1997), Hendriks et al. (1996) and Sritharan (1998).

The impact of these higher endogenous protein and amino acid excretions from the intestines of cats in response to dietary protein needs to be taken into account in diet formulation. The minimum amino acid requirement estimates as presented by the NRC (1986) have been determined using free amino acid diets. It is now well known that free amino acid diets, like protein-free diets, significantly underestimate endogenous amino acid excretion from the intestine. More dietary protein therefore is required for the replacement of these losses.

Furthermore, there may be antinutritional factors either present in the dietary ingredients or formed upon the required heat treatment of pet foods that may further increase endogenous amino acid losses from the intestine of the cat. Dietary fibre and antinutritional factors (e.g. trypsin inhibitors, tannins, lectins) have been shown to significantly affect endogenous amino acid losses in other animal species (Souffrant, 2001; Lange et al., 2001).


Measurement of protein digestibility in cats

Pet foods are formulated using a variety of ingredients such as meats/offals (beef, fish, poultry, deer, lamb, etc.), cereal grains, meat by-products, fats/oils, vegetable protein concentrates, sugar, water, humectants, gelling agents, emulsifiers, colourants, vitamins and minerals. The large number of ingredients used contributes to the observed variability in the nutritional content. To increase shelf life, achieve a desired physical form, and(or) increase palatability, the unprocessed mixture is either extruded, baked, pasteurised or sterilised depending on the pet food type.

Digestion and absorption of dietary and endogenous amino acids are integral parts of protein metabolism. The protein content and digestibility of the ingredients in pet foods contributes to the overall digestible protein and amino acid content of the final product. Typical apparent faecal digestibility values for moist and dry cat foods are in the order of 75- 85% (Kendall et al., 1982).

An ingredient such as guar gum, which is frequently used in a variety of pet foods, has been shown to significantly decrease the apparent faecal digestibility of crude protein in moist cat foods. In a study by Harper and Siever- Kelly (1997), adult cats aged 1.5 to 13 years were fed two diets differing in amounts of guar gum. In addition to the reduction in protein digestibility due to guar gum inclusion, protein digestibility was shown to decrease with age.

Many of the premium foods for cats and dogs also report digestibility values determined using the faecal digestibility methodology such as described by AAFCO (2000). However, how accurate are these reported digestibility values of dietary crude protein? The main reason for measuring the digestibility of a nutrient is to obtain an estimate of the amount of a dietary nutrient the animal can absorb and potentially use for metabolic purposes.

There are convincing data in other animal species (e.g. pigs, rats, humans, chickens) that dietary and endogenous protein and amino acids are metabolised by the microflora residing in the large intestine, resulting in inaccurate digestibility estimates for protein and amino acids (Butts et al., 1991). Even though this organ is relatively short and ‘undeveloped’ in the cat and dog compared to other animal species (Morris and Rogers, 1989), a significant metabolism of dietary nutrients seems to occur.

Differences in the digestibility of crude protein between the small and large intestine of dogs of +5.7 to +17.1% (Muir et al., 1996), +7.0 to +14.2% (Murray et al., 1998) and +8.5% (Sritharan, 1998) have been reported. Faecal digestibility values overestimate the digestibility of dietary protein in dogs by approximately 10%. Like other animals, the apparent faecal digestibility technique appears to be an inaccurate method for the estimation of protein absorption in dogs, in spite of the fact that the large intestine is a relatively ‘undeveloped’ organ in dogs. Yet it seems to have a significant influence on the determination of protein absorption in this species.

There are currently no data available on the influence of the large intestine on nutrient digestibility in cats. Although the large intestine of the cat is even less developed compared to dogs, published data for dogs show that this criterion cannot be taken as evidence that dietary nutrients are not metabolised by the microorganisms in the large intestine. Realisation of the overestimation of the protein digestibility by the faecal apparent digestibility method needs to be taken into account in the formulation of diets for cats.

Other inaccuracies in formulation can originate from the use of the nitrogen content of diets to calculate the protein content. Crude protein is defined as nitrogen times a factor of 6.25 as it is assumed that the nitrogen content of protein is generally 16%, like egg protein.

However, the nitrogen content of protein can vary from as little as 4.2 up to 30% depending on the amino acid composition (Pomeranz and Meloan, 1980). For protein that contains amino acids with multiple nitrogen atoms (e.g. lysine, histidine, arginine, tryptophan), the percentage of nitrogen in the protein is higher and as a result the multiplier that should be used is less than 6.25. Crude protein therefore may not be an accurate estimate of the true protein content of the diet and can as a result affect protein digestibility estimates.

Taking into account the previously mentioned inaccuracies in protein digestibility values, underestimation of protein requirements, and increased requirements of protein for felinine synthesis, a protein content of less than 20-24% of the dry matter in commercial diets is likely to be insufficient to meet the protein requirements of adult cats unless highly digestible proteins are used.


Heat processing of feline diets

It is well known that the processing of foods and ingredients may result in damage to protein and that certain amino acids (e.g. lysine, methionine) are particularly susceptible to heat damage. As pet foods are extensively heat processed, changes in the digestibility and availability of protein in general and of certain amino acids can be expected.

Extrusion cooking, used to produce dry diets for cats and dogs, is well known for causing damage to amino acids, especially lysine, with the extent of the damage depending on the transit time and moisture content of the unprocessed diet and the heat applied (Cheftel, 1986; Hendriks et al., 1994).

Little research has been conducted to date on the heat damage to nutrients in pet foods, although it is believed that the heat processing normally employed negatively affects the nutritive value of the food. Backus et al. (1995), however found an increase in the faecal nitrogen digestibility of cats after heat-sterilisation of a canned commercial moist cat food. This seems to indicate that heat processing may have a beneficial effect by increasing crude protein digestibility.

However, as previously mentioned, the faecal digestibility of nitrogen is not an accurate measure of the absorption of protein from the diet. The cats fed the heat-treated diet may have had a decreased deamination of dietary amino acids by microorganisms in the large intestine, which result in a higher faecal digestibility of nitrogen. In diets that have undergone processing or prolonged storage, the ε-amino group of lysine can react with compounds present to produce nutritionally unavailable derivatives such as deoxyketosyllysine.

This compound, formed during the early stages of the Maillard reaction, is partially absorbed from the gut but is of no nutritional value to the animal (Hurrell and Carpenter, 1981). However, during the acid hydrolysis step in amino acid analysis, deoxyketollysine partly reverts back to lysine and could lead to overestimation of the lysine content of diets and the digestibility assay will generally overestimate the lysine availability in heat-treated diets (Hodgkinson and Moughan, 2000).

Determination of unmodified lysine (reactive lysine) in diets will provide a more accurate measure of the potential available lysine present. In addition, determination of reactive lysine before and after heat processing may present a useful in vitro method to assess heat-damage to protein before destruction of amino acids is observed.

Rutherfurd and Moughan (1997) measured the digestibility of total and reactive lysine (available) in a moist and dry cat food. These authors found no difference between the total digestible and available lysine for the dry cat food (produced by extrusion) indicating that lysine was not present in a form that reverts back under acid hydrolysis conditions. There was a 10% difference in the digestibility of total and reactive lysine in the moist cat food indicating that heat damage to lysine may have occurred.

Recently Hendriks et al. (1999) measured the effect of heat sterilisation on the protein quality of a canned cat food mixture using an in vivo assay and several in vitro assays. A standard recipe cat food was heat treated for different times and analysed for crude protein, amino acids, reactive lysine and fed to rats to determine the true ileal digestibility of amino acids. No changes were observed in the gross composition of nutrients (crude protein, amino acids and reactive lysine) between the different treatments indicating that amino acids are not destroyed with the normal heat-sterilisation processes used in the manufacture of canned pet foods.

However, the true ileal digestibility of amino acids decreased with increasing heat treatment (Figure 2). Amino acid nitrogen and proline digestibility increased with mild heat treatment but decreased with more severe heat processing. The digestible proline and glycine content were higher in all the heat-treated diets compared to the untreated diet. This increase in digestibility of proline and glycine indicates that there may be benefits from the heat processing of moist diets.

The reactive lysine content in the unheated and heattreated diets was found to be about 10% lower than the total lysine content, a similar value to that found by Rutherfurd and Moughan (1997). The lower reactive lysine value in the study by Hendriks et al. (1999) was attributed to the relatively high proportion of collagen (from connective tissue) present in the diet that naturally contains covalent cross-links involving lysine to maintain the native threedimensional structure of the protein. It appears that cross-linking involving cysteine is the most likely mechanism for the decrease in the true digestibility of amino acids with increasing heat treatment.

Figure 2. True ileal digestibility of lysine, threonine, glycine and amino acid nitrogen in a moist cat food heated to different lethality values (time equivalent of a heating process to destroy microorganisms at the reference temperature of 121.1°C).

References

AAFCO. 2000. Official Publication of the Association of American Feed Control Officials Inc., Atlanta, GA, USA.

Backus, R.C., Q.R. Rogers, G.L. Rosenquist, J. Calam and J.G. Morris. 1995. Diets causing taurine depletion in cats substantially elevate postprandial plasma cholecystokinin concentration. J. of Nutr. 125:2650-2657.

Beauchamp, G.K., O. Maller and J.G. Rogers, Jr. 1977. Flavor preferences in cats (Felis catus and Panthera sp.). J. of Comp. and Physiol. Psychol. 91:1118-1127.

Boudreau, J.C. and T.D. White. 1978. Flavor chemistry of carnivore taste systems. In: Flavour Chemistry of Animal Foods (R.W. Bullard, ed.), American Chemical Society, Washington DC, pp. 102-128.

Butts, C.A., A.J. Darragh, and P.J. Moughan. 1991. The protein nutrition of simple-stomached mammals, birds and fishes. Proc. of the Nutr. Soc. of New Zealand 16:60-81.

Cheftel, J.C. 1986. Nutritional effects of extrusion cooking. Food Chem. 20:263-283.

Datta S.P. and H. Harris. 1951. A convenient apparatus for paper chromatography. Results of a survey of the urinary amino acid patterns of some animals. J. of Physiol. 114:39-41.

Greaves, J.P. 1965. Protein and caloric requirements of the feline. In: Canine and Feline Nutritional Requirements. (O. Graham-Jones, ed.), Pergamon Press, Oxford, England, pp. 33-45.

Harper, E.J. and C. Siever-Kelly. 1997. The effect of fibre on nutrient availability in cats of different ages. In: Recent Advances in Animal Nutrition in Australia 1997, (J.L. Corbett, M. Choct, J.V. Nolan and J.B. Rowe, eds.), University of New England, Armidale, Australia, pp. 110-116.

Hendriks, W.H., P.J. Moughan, H. Boer and A.F.B. van der Poel. 1994. Effects of extrusion on the dye-binding, fluorodinitrobenzene-reactive and total lysine content of soyabean meal and peas. Ani. Feed Sci. and Technol. 48:99-109.

Hendriks, W.H., M.F. Tarttelin and P.J. Moughan. 1995. Twenty-four hour felinine excretion patterns in entire and castrated cats. Physiol. and Behav. 58:467-469.

Hendriks, W.H., P.J. Moughan and M.F. Tarttelin. 1996. Gut endogenous nitrogen and amino acid excretions in adult domestic cats fed a proteinfree or an enzymatically hydrolyzed casein-based diet. J. of Nutr. 126:955-962.

Hendriks, W.H., M. Emmens and J.R. Pluske. 1999. Heat processing changes the protein quality of canned cat foods as measured by a rat-bioassay. J. of Ani. Sci. 77: 669-676.

Hendriks, W.H., S.M. Rutherfurd and K.J. Rutherfurd. 2001. Relative importance of 35Smethionine, 35S-cysteine and 35S-sulphate in the synthesis of felinine. Comp. Biochem. and Physiol. 129:211-216.

Hodgkinson, S.M. and P.J. Moughan. 2000. Amino acids: digestibility, availability and metabolism. In: Feed Evaluation Principles and Practice. (P.J. Moughan, M.W.A. Verstegen and M.I Visser- Reyneveld, eds), Wageningen Press, Wageningen, The Netherlands, pp. 125-132.

Hurrell, R.F. and K.J. Carpenter. 1981. The estimation of available lysine in feedstuffs after Maillard reaction. In: Progress in Food and Nutritional Science. Maillard Reactions in Food. (L. Eriksson, ed.), Volume 5, No. 1-6, Pergamon Press, Oxford, United Kingdom, pp. 159-176.

Kendall, P.T., D.W. Holme, and P.M. Smith. 1982. Comparative evaluation of net digestive and absorptive efficiency in dogs and cats fed a variety of contrasting diet types. J. of Small Ani. Prac. 23:577-587.

Lange, C.F.M. de, C.M. Nyachoti and M.W.A Verstegen. 2001. The significance of antinutritional factors in feedstuffs for monogastric animals. In: Feed Evaluation Principles and Practice. (P.J. Moughan, M.W.A. Verstegen and M.I Visser- Reyneveld, eds), Wageningen Press, Wageningen, The Netherlands, pp. 169-188.

Morris, J.G. 2001. Unique nutrient requirements of cats appear to be diet-induced evolutionary adaptations. Rec. Adv. in Ani. Nutr. in Aus. 13:187-194.

Morris, J.G. and Q.R. Rogers. 1989. Comparative aspects of nutrition and metabolism of dogs and cats. In Nutrition of the Dog and Cat. (I.H. Burger and J.P.W. Rivers, ed.), Cambridge University Press, Cambridge, pp. 35-66.

Moughan, P.J. 1997. Nutrition, science and comparative biology. Proc. of the Nutr. Soc. of New Zealand 22:1-19.

Muir, H.E., S.M. Murray, G.C. Fahey, Jr., N.R. Merchen and G.A. Reinhart. 1996. Nutrient digestion by ileal cannulated dogs as affected by dietary fibers with various fermentation characteristics. J. of Ani. Sci. 74:1641-1648.

Munro, H.N. 1964. An introduction to biochemical aspects of protein metabolism. In: Mammalian Protein Metabolism. (H.N. Munro and J.B. Allison, ed.), Volume I, Academic Press, New York, pp 31-34.

Murray, S.M., A.R. Patil, G.C. Fahey, N.R. Merchen and D.M. Hughes 1998. Raw and rendered animal by-products as ingredients in dog diets. J. of Nutr. 128:2812S-2815S.

NRC 1986. Nutrient Requirements of Cats. Revised edition, National Academy Press, Washington DC.

Oomori, S. and S. Mizuhara. 1960. Structure of a new amino acid isolated from the urine of hypercholesterolemic patients. Biochem. and Biophys. Res. Com. 3:343-345.

Pomeranz, Y. and C.E. Meloan. 1980. Nitrogenous compounds. In: Food Analysis: Theory and Practice. (Y. Pomeranz and C.E. Meloan, ed.), revised edition, AVI Publishing Company, Westport, Connecticut. pp. 668-691.

Rutherfurd, S.M. and P.J. Moughan. 1997. Determining reactive lysine and the digestibility of reactive lysine in heat processed foods. Proc. of the Nutr. Soc. of New Zealand 22:222-234.

Rutherfurd, K.J., S.M. Rutherfurd, P.J. Moughan and W.H. Hendriks. 2001. Isolation and characterisation of a felinine containing peptide from the blood of the domestic cat (Felis catus). J. Biol. Chem. 277:114-119.

Souffrant, W.B. 2001. Effect of dietary fibre on ileal digestibility and endogenous nitrogen losses in the pig. Ani. Feed Sci. and Technol. 90:93-102.

Sritharan, K. 1998. The Rat as a Model Animal for Digestion in the Dog. MSc thesis, Massey University, Palmerston North, New Zealand.

Ubuka, T.K. Horiuchi, T. Shimomura and S. Mizuhara. 1964. Experimental isovalthinuria IV. Incorporation of S35-methionine or S35-cystine into urinary isovalthine. Acta Med. Okayama 18:65- 70.

Woolhouse, A.D., D.R. Engelbretson, D.R.K. Harding, W.H. Hendriks, S.M. Rutherfurd and K.J. Rutherfurd. 2002. Synthesis of the L-felininecontaining peptide for the blood of the domestic cat (Felis catus) Tet. Let. (submitted).

Readers' Rating:

Rate this article

Send to a friend