Dissociation of ‘hybrid’ isozymes on electrophoresis

Abstract

HETEROMERIC or ‘hybrid’ isozymes can be readily demonstrated by electrophoretic techniques. They can be observed, for example, in individuals who are heterozygous for alternative alleles at an autosomal locus determining a multimeric enzyme or where multiple loci are involved in the determination of a multimeric enzyme. Heteromeric forms can also be generated by techniques such as somatic cell fusion, and by dissociation and recombination in vitro. The number of heteromeric isozymes displayed by a particular enzyme is related to its subunit composition. For example, in a heterozygote a dimeric enzyme exhibits one heteromeric isozyme and two homomeric isozymes, whereas a tetrameric enzyme displays three heteromeric isozymes in addition to the two homomeric forms. Thus, the number of heteromeric isozymes provides a valuable method of assessing the number of subunits making up active enzyme molecules and indeed this approach has recently been used in a survey of the subunit structures of 66 different human enzymes¹. It was found that heteromeric isozymes usually occur as described here, but there are occasional exceptions where the expected ‘hybrid’ isozymes are unusually weak or absent. We have investigated one component of the electrophoretic separation system—that is, temperature—and its influence on the occurrence of heteromeric isozymes using soluble superoxide dismutase (SOD_S) as a test case. This was prompted by a report that this dimeric enzyme is readily dissociated at high temperatures²; also, the heteromeric isozymes of SOD_S can be conveniently studied by electrophoretic methods. Our results suggest that temperature has a crucial effect on the occurrence of the heteromeric isozymes of this enzyme.