END-PRODUCT INHIBITION OF ASPARTATE TRANSCARBAMYLASE IN VARIOUS SPECIES

The activity of aspartate transcarbamylase (ATCase) from Escherichia coli is known to be regulated by feed-back inhibition. The catalytic and end-product inhibition characteristics of this enzyme from various species were compared. The enzyme from lettuce seedlings is best inhibited by uridylic acid. The inhibition is noncompetitive with regard to both aspartate and carbamyl phosphate, the substrates for the enzyme; it can be relieved specifically by ribose-5-phosphate without enzyme activity being impaired. The ATCase from Pseudomonas fluorescens is inhibited by various pyrimidine and purine derivatives, both in crude and 50-fold purified extracts. The most potent inhibitor is uridine-5′-triphosphate (UTP) but adenosine-5′-triphosphate also inhibits strongly. The degree of inhibition by UTP increases with decreasing carbamyl phosphate concentration and is independent of aspartate concentration. The enzyme could not be “desensitized” toward the inhibition by UTP by a variety of treatments without a parallel decrease in enzyme activity. ATCase from Bacillus subtilis is not inhibited by a large number of pyrimidine derivatives. The enzyme is inhibited by phosphate (product inhibition) competitively with regard to CAP. In this bacterium the synthesis of the ATCase is regulated by feed-back repression. The possible meaning of the differences in the characteristics of ATCase from the various organisms was discussed.