. on eern BeBe Nm aba me Porp/Baiuisr/UirnseH/CoNRAD 210 Because human hemoglobin A, in comparison with most other proteins, has an unusual chemical property, i.e. it contains no coded isoleucine [3], we believed that chemical methods alone could be used to determine the substitution frequency of isoleucine for other amino acids in highly purified hemoglobin fromindividuals of various ages. An increase in the isoleucine substitution frequency with age could be evidence in support of the érror theory of apeing. However, the small quantity of isoleucine found in hemoglobin A can be incorporated by more than one mechanism. These are: (i) infrequent errors in the transcription of DNA to mRNA may change nonisoleucine codons into isoleucine codons; (2) errors in transcription may produce altered forms of tRNA; these may have reduced fidelity as regards both the kinds of amino acids they accept and the species of mRNA codons they recognize: (3) errors in translation of the correct mRNA can occur through mispairing of tRNA anticodons and their proper mRNA codons; (4) amino acyl synthetase errors can cause the wrong amino acid-tRNA complexes to be formed, and (5) mistakes in the replication of DNA involving the exchange of appropriate base pairs [2] lead to the formation of mutant cells whose hemoglobin mRNAscontain isoleucine codons. Chemical determinations, as a composite measure of these errors, of the frequency at which isoleucine substitutes for other amino acids in hemoglobin of normal! adults, ages 20-51, are reported in this paper. Materials and Methods Blood donors were composed of 3 laboratory personnel, 12 Marshall Island residents, and 13 Marshall Island residents exposed to y-radiation fallout in 1954; the latter were positive controls. 10 ml of blood from each individual was collected ina heparinized syringe. Erythrocytes were separated from plasina by centrifugation for 10 min at 600 g and washed 3 times in 10 vot of saline. Samples from the Marshallese were resuspended in an equal volume of phosphate-buffered saline and shipped on ice by air and washed again upon. arrival. The packed red cells were lysed by adding 4 vol of cold distilled water. Red! cell stroma was removed by centrifugation for 15 min at 20,000 g. Hemoglobin in the supernatant was converted to the carbon monoxide form by bubbling with CO in a hood; one drop of octanol was added to prevent foaming. The hemoglobin solutions were kept cold during all subsequent processing (cold room at 4°C). IOl2b15 | a Jt? ‘ TNR me ee oy ‘