WALTHAM International Science Symposium: Nature, Nurture, and the Case for Nutrition Urinary Isovalthine Excretion in Adult Cats Is Not Gender Dependent or Increased by Oral Leucine Supplementation

Domestic cats and other Felidae species are known to excrete three unusual amino acids in their urine; felinine, isovalthine, and isobuteine. Felinine (2-amino-7-hydroxy-5, 5-dimethyl-4-thiaheptanoic acid) was discovered in 1951 and is unique to certain Felidae species including the domestic cat (1). Isovalthine (2-amino-5-carboxy-6-methyl-4-thiaheptanoic acid) was found in the urine of healthy domestic cats (2), lions (3), and humans suffering from hypercholesterolemia (4), and can be induced in a number of animal species including guinea pigs, dogs, rats, and rabbits (5). Information on isobuteine (2amino-6-carboxy-4-thiaheptanoic acid) is limited to the isolation of this amino acid from the urine of healthy humans and cats, and its characterization and structural confirmation by synthesis (4). The biological roles of felinine, isovalthine, and isobuteine remain unknown. Felinine was hypothesized to be a precursor to a pheromone, and although much circumstantial evidence is available, this remains to be proven. The biological role of isovalthine and isobuteine, if indeed they have a biological function, remains a matter for speculation. The carbon backbone of the side chain of both felinine and isovalthine appears to be derived from isoprenoid units similar to that used to synthesize cholesterol. Consequently, both isovalthine as well as felinine have at various times been hypothesized to be involved in cholesterol regulation (1). However, because felinine excretion was shown to be gender dependent (6), the latter role seems unlikely. There is little information available on normal excretion levels of isovalthine in domestic cats. One report (1) found that administration of leucine to cats resulted in increased urinary isovalthine excretion. The aim of this study was to determine normal daily urinary excretion levels of isovalthine in adult domestic cats of different genders, and the effect of oral leucine supplementation on daily urinary isovalthine excretion in male, female, and castrated cats.