The amounts of endogenous protein and amino acids lost through the urinary tract are considered to be small in comparison with the losses of endogenous nitrogen material from other routes (Linder, 1991). Reabsorption of free plasma amino acids by the kidney is a relatively efficient process and only small amounts of free amino acids can be found in the urine. The exceptions are those amino acids being excreted as a waste product (e.g. 3-methylhistidine) or amino acids excreted which have a biological role outside the body (e.g. felinine). In addition to these losses, small amounts of endogenous protein can be found in the urine originating from sloughed cells, mucus, enzymes, plasma proteins, etc.
Quantitative data on the losses of protein and amino acids through the urinary tract in adult cats and dogs are limited. In healthy adult dogs, 24 h urinary protein excretions of 4.8, 8.2 and 13.9 mg kg-1 body weight have been reported (DiBartola et ai, 1980; Biewenga et ai, 1982; White et ai, 1984). Similar values have been reported by Monroe et al. (1989) who found that entire male cats excreted on average 8.7 mg protein kg-1 body weight day-1 whereas entire female cats excrete significantly more (16.6 mg kg-1 day-1). Lower values of 4.9 mg kg-1 day-1 were found by Adams et al. (1992) in adult cats White Russo et ai (1986) reported a value of 17.4 mg-1 day-1 and found no significant differences between male and female cats. Hendriks et al. (1997a) determined the endogenous content of several urinary nitrogen fractions in adult cats fed a protein-free diet and by regression to zero dietary nitrogen intake, and found undetermined nitrogen (total N-urea N-creatinine N-ammonia N) fractions of 57.6 and 38.0 mg kg-1 day-1, respectively. The undetermined nitrogen fraction would have contained free amino acids, mucus, sloughed cells, enzymes, creatine, metabolites of purine metabolism (uric acid, allantoin, xanthine, hypoxanthine), etc.
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