Asparagine metabolism in the rat: Study of the relative importance of L-asparaginase and asparaginase II in the in vitro catabolism of asparagine

Abstract

Analytical methods were developed to study asparagine metabolism in the rat by: 1) distribution of C???ü? from catabolism of U???ü? C-L-asparagine and U???ü? C-L-aspartic acid 2) response to dietary asparagine of Iiver L-asparaginase and asparaginase II activities as measured from ammonia production 3) response of weight gain and gain/feed to dietary asparagine 4) response of free asparagine pools to dietary asparagine. The distribution of C???ü? in the in vitro catabolism of U???ü? C- L-asparagine and U???ü? C-L-aspartic acid, in response to 1) heat denaturation at 50??C for 10 minutes 2) presence of L-cycloserine 3) subcellular composition of liver supernatants, indicated that under the imposed reaction conditions (no exogenous pyruvic acid added to the incubation tube) the L-asparaginase reaction was the primary route of asparagine catabolism. In separate studies, it was found that liver L-asparaginase activity of growing rats was significantly affected (P < .01) by days on test (1-13) and dietary asparagine level (0.0%, 0.6%. and 4.0%), but asparaginase II activity was not significantly affected by either. Compared to the L-asparaginase activity of rats fed the asparagine-free diet, feeding 0.6% asparagine over the 13 day period increased L-asparaginase activity 39%. Four percent dietary asparagine did not cause further increase in L-asparaginase.activity. On day 13, the asparaginase II activity of rats fed the asparagine-free diet (2.8 nmoles NH?éâ/mg protein/min) was significantly greater (P < .05) than the asparaginase II activity of rats fed a 4.0% asparagine diet (1.2 nmoles NH?éâ/mg protein/min). The proportion of total asparagine catabolized by asparaginase II activity was 36%, 23% and 13%, respectively, for rats fed 0.0%, 0.6%, and 4.0% dietary asparagine..