The distributions of subunit numbers and subunit sizes of enzymes: a study of the products of 100 human gene loci.

1. A tabulation of subunit numbers and subunit sizes of a series of enzymes which have been studied electrophoretically in man is presented. The series of subunit numbers cover the isozyme products of 100 distinct gene loci. For 99 of these, estimates of subunit size are given. 2. The distribution of subunit numbers in the whole series is as follows: monomers, 28; dimers, 43; trimers, 4; tetramers, 24; octamers, 1. 3. The subunit sizes range from 13,000 to 116,000. The average subunit size for the whole series is close to 45,800. This corresponds to an average polypeptide chain length of about 425 amino acids. 4. No significant differences were found between the average subunit sizes for the separate classes of enzyme with different subunit numbers. 5. The enzymes were categorized in six different types according to the classification of the International Enzyme Commission. It was found that the oxido-reductases differed from the other types (transferases, hydrolases, lyases, isomerases, ligases) in showing a much greater proportion of multimeric enzymes. Only 1 out of 24 oxido-reductases appeared to be a monomer, whereas in the other enzyme types approximately one-third of the enzymes considered appeared to be monomers. No significant differences in subunit size were found between the various enzyme types. 6. It was found that, in general, where two or more separate gene loci are concerned in determining sets of isozymes with the same or very similar enzyme characteristics, there is a close correlation in subunit size and in most, though not all, cases the subunit numbers are the same.