l‐Ornithine carbamoyltransferase and arginase from Saccharomyces cerevisiae have been extensively purified and their quaternary structures have been determined by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate. Subunit cross‐linking with glutaraldehyde shows that both enzymes have trimeric structures. Ornithine carbamoyltransferase is built up by the association of three subunits of molecular weight 37500; arginase is made of three subunits of 39000. The latter observation has lead us to reinvestigate the quaternary structure of rat liver arginase for which, in spite of an earlier report to the contrary [H. Hirsch‐Kolb and D. Greenberg, J. Biol. Chem. 243, (1968) 6123–6129], evidence of a trimeric structure is also obtained. The data suggest that the 1‐to‐1 regulatory complex of ornithine carbamoyltransferase and arginase of S. cerevisiae, leading to ornithine carbamoyltransferase inhibition, is an hexamer.

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Yeast argininosuccinate synthetase has been purified to homogeneity. The enzyme was found to have a molecular weight of 228000 as determined by gel sieving. It is composed of identical subunits of M , 49000 as shown by gel electrophoresis.The quaternary structure as determined by cross-linking of the subunits with glutaraldehyde, followed by gel electrophoresis with dodecylsulfate, is tetrameric.The saturation functions by citrulline and aspartate are hyperbolic ; with MgATP as the variable substrate a sigmoid character, dependent on the concentration of citrulline, aspartate, argininosuccinate and arginine, was observed. The positive cooperativity is reduced by increasing concentrations of citrulline and aspartate ; it is increased by argininosuccinate and arginine.Kinetic analysis provided evidence for a random addition of substrates. Initial velocity studies as well as product and dead-end inhibition studies comply with a rapid-equilibrium random model, except for the interconversion of the central quaternary complexes ; the different kinetic constants have been established on this basis.Yeast arginonosuccinate synthetase has a double metabolic function : anabolic in the biosynthesis of arginine, catabolic as the first enzyme of citrulline utilization as nitrogen source. The kinetic properties of the enzyme point to a physiologically well-adjusted activity for both roles and to an economic and efficient utilization of ATP.Argininosuccinate synthetase catalyses the penultimate reaction of the arginine biosynthetic pathway :Citrulline + aspartate + ATP + Argininosuccinate + AMP + pyrophosphate.