Effect of age on activities of urea cycle enzymes in the bovine

Eighteen Hereford cows averaging 429.6 +/- 14.5 kg in weight, ranging from 3.33 to 12.12 years of age and consuming 18.2 kg corn silage/animal/day were used to observe the effect of age on activities of the liver enzymes of urea synthesis. Liver samples were taken by aspiration biopsy and homogenized in water for enzyme assay. Plasma urea-nitrogen of all cows averaged 4.20 mg per 100 ml. The enzymes measured were: carbamyl phosphate synthetase (CPS), ornithine transcarbamylase (OTC), argininosuccinate synthetase (AS), argininosuccinase (AL) and arginase (A). Mean enzyme activities of the cows were: CPS 1.57, OTC 44.4, AS 0.61, AL 1.05 and A 203.9 mum product/mg protein/hr. Regression coefficients (b) of enzyme activity (units) on age (years) were 0.021 +/- .018, 0.713 +/- .379, 0.016 +/- .016, 0.032 +/- .014 and 6.73 +/- 5.09 units for the five enzymes, respectively. Age (3.3 to 12.1 years) did not significantly (P less than .05) affect activities of the urea cycle enzymes (per mg protein, per g liver or per W-75 kg/hr) or plasma urea-nitrogen levels in the adult Hereford female at the level of dietary protein fed in this study.