Effect of vitamin A deficiency on nitrogen balance and hepatic urea cycle enzymes and intermediates in rats.

After 12 wk of feeding a 4-d nitrogen balance was carried out in 8 vitamin A--deficient and 8 pair-fed control rats to understand the effect of vitamin A deficiency on protein metabolism. Urea nitrogen (UN) was lower and amino nitrogen (AN) was higher in plasma of deficient animals than in pair-fed controls. No significant alteration in the nitrogen retention or in the general pattern of other nitrogen metabolites in plasma and urine was observed in vitamin A--deficient rats as compared to controls. However, there was a significant increase in the excretion of urinary ammonium nitrogen (Am-N) in relation to creatinine (CR). Activities of hepatic carbamoylphosphate synthase-I (CPS-I) and ornithine transcarbamoylase (OTC) were significantly lower in vitamin A--deficient animals than in control rats at the end of 13 wk of feeding. While the liver levels of ornithine and polyamines were significantly greater, that of glutamine was lower in vitamin A--deficient rats than in pair-fed controls. The results suggest that vitamin A deficiency leads to a reduced efficiency of urea synthesis pathway, thus accounting for the increased Am-N excretion seen in vitamin A deficiency.