Partially purified Bacillus subtilis ornithine carbamoyltransferase is cooperatively inhibited by excess of ornithine. The inhibition is decreased by lowering of pH from 9.2 to 7.6 without altering the affinity of the enzyme for the neutral species of ornithine which seems to be its true substrate.Arginine and lysine partially reverse this inhibition without affecting the catalytic activity of the enzyme. 2-Aminobutyrate, an inhibitor of the enzyme, competitive with respect to ornithine, is not able to replace ornithine in this excess-substrate type of inhibition. Zn2+ is also an inhibitor of ornithine carbamoyltransferase, with competitive action towards ornithine and its inhibition is added to the excess ornithine inhibition. These data suggest that ornithine carbamoyltransferase has a regulatory site for ornithine, distinct from the catalytic one.Partially purified arginase, prepared from the same strain, increases the inhibition of ornithine carbamoyltransferase by excess of ornithine when arginine is present. An association of the two enzymes is suggested by molecular sieving. These data postulate that ornithine carbamoyltransferase is regulated by arginase under the control of arginine and ornithine. The other substrate, carbamoyl phosphate, is not involved in this regulatory mechanism.Lysine and Zn2 + inhibit arginase competitively with respect to arginine without altering the effect of this enzyme on the inhibition of ornithine carbamoyltransferase. 2-Aminobutyrate inhibits arginase non-competitively with respect to arginine and prevents the inhibition of arginase towards ornithine carbamoyltransferase. These results suggest that arginase possesses a regulatory site for arginine, distinct from the catalytic one, which is also involved in the control mechanism.Wiame et al. [l-31 have uncovered a novel mechanism of metabolic regulation in the arginine cycle of yeasts. This mechanism, for which the name "epienzymatic control" has been coined [2], results from the binding of ornithine carbamoyltransferase with arginase, under the control of arginine and ornithine, forming a complex in which the ornithine carbamoyltransferase is strongly inhibited without substantial modification of the arginase activity. This control mechanism ensures that the arginine cycle will not spin aimlessly, making and destroying arginine with a net loss of energy.Aas a step towards exploring the generality of epienzymatic control mechanism, it appeared of interest to establish whether they also occur in prokaryotes. The present study will show that, indeed, the ornithine carbamoyltransferase of Bacillus subtilis is Enzymes.

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