’ es 0 oN tae may + er.’ : . (fiedade BS aaLede Errors in Human Hemoglobin as a Function of Age 215 was obtained from a 60-year-old woman showing signs of senility. Little significance can be attached to this sample taken from the oldest person in this study; however, it docs suggest that more data on older persons should be obtained to determine whether the isoleucine substitution frequency increases linearly or has an exponential component during advanced ageing [9]. As stated in the Introduction, isoleucine incorporation into human hemoglobin A may originate from genetic and/or nongenetic errors. One approach toward estimating the contribution of somatic mutations to the average isoleucine substitution frequency would be to assume that the mutation frequencies due to polymerase errors in germinal and somaticcells are similar. This may be a valid assumption for comparing spermatogonia and erythropoietic stem cells; both divide continually and differentiate into terminal cells. However, oogonia, which remain at prophase of the second meiotic division, may have a lower frequency of mutations owing to polymer- ase errors. The rate of germinal mutations in man has been estimated from the frequency of occurrence of inherited genetic diseases, which suggests an average mutations frequency of 1 x 10-5/locus/generation. It has been argued that this estimate is too high, but a higher value based on molecular changes within proteins has been calculated recently [8]. Most inherited genetic diseases result from defective enzymatic mechanisms. Mutations within the active centers of enzymes (usually less than 5% of the molecule) cause inactivation or altered substrate specificity sufficiently different from normal to produce an altered phenotype, whereas mutations at amino acid residues outside the active center generally do not markedly change the specificity of an enzyme. Isoleucine is 1 of 20 amino acids that substitute for other amino acids but since only 5% of all mutations randomly occurs within an active site, one deduces that an isoleucine substitution for other amino acids occurs at approximately the same frequency as the appearance of expressed mutations, i.e. about 1 x 10-*/locus/generation. Thus, this portion of the total isoleucine substitution frequency may be due to mutations while the remainder, 2x 10-5, may result from nongenetic error. A better estimation of the contribution of somatic mutations and nongenetic errors toward thetotal isoleucine substitution frequency can be made in an experimental animal with the aid of isotopically labelled amino acid incorporation followed by sequential degradation of the polypeptide. Using such procedures, the absolute substitution frequencies of isoleucine for the various amino acids in the polypeptide can be determined. For amino acids whose codons can be changed from nonisoleucine codons into isoleucine codons by a single base substitution, e.g., GUU (Val) to AUU (He), the