Hybridization of Native and Chemically Modified Enzymes, III. The Catalytic Subunits of Aspartate Transcarbamylase

Succinylation of the catalytic subunits of ATCase yielded a relatively homogeneous, inactive electrophoretic variant which upon mixing with native regulatory subunits formed a complex the size of the native enzyme. Hybridization experiments with mixtures of this variant and the native catalytic subunits in the presence of excess regulatory subunits yielded three different molecular complexes which were separated and individually characterized. The number and properties of the various components indicated that each ATCase molecule contains two catalytic subunits. Hybridization was also effected at the intrasubunit level by dissociation and reconstitution of mixtures of the native and modified catalytic subunits. These experiments produced four components showing thereby that each catalytic subunit is composed of three polypeptide chains. The potential use of the various hybrids is discussed in relation to the unique properties manifested by regulatory enzymes.