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Reconstitution of aspartate transcarbamoylase (EC 2.1.3.2) from dilute solutions of the isolated regulatory and catalytic subunits, with the latter in large excess, led to the formation of appreciable amounts of a second, stable component in addition to the reconstituted enzyme. The purified component, designated r(4)c(6), was found to have a molecular weight about 3 x 10(4) less than that of the native enzyme, and it combined with isolated regulatory subunit to form aspartate transcarbamoylase. It also combined with one succinylated regulatory subunit to form a hybrid species that was identified electrophoretically. These findings indicate that r(4)c(6) differs from the native enzyme in that only two (rather than three) regulatory subunits participate in "crosslinking" the two catalytic trimers. The "incomplete" enzyme, r(4)c(6), exhibits the characteristic sigmoidal saturation behavior and CTP inhibition of aspartate transcarbamoylase; however these allosteric effects are reduced in extent by about one-third in comparison to the native enzyme and free catalytic subunits. The complex, which may be an intermediate in the assembly and dissociation of the native enzyme, is useful in assessing the role of the various bonding domains responsible for the stability and regulatory properties of the native enzyme.
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