Comparative Radiolability of Amino Acids of Proteins and Free Amino Acids

Abstract

Despite considerable work in the field of radiation chemistry of amino acids and proteins, the mechanisms of radiation damage to free amino acids and amino acids of proteins are not well understood, and more knowledge of the generalities of comparative radiolabilities of free amino acids and amino acids of proteins is needed. Drake et al. (1) determined the radiolability of amino acids of insulin in pH 3.0 solution and found that cystine, tyrosine, phenylalanine, proline, and histidine were the most radiolabile. Recently, we determined the radiolability of amino acids of cytochrome c, hemoglobin, and ovalbumin at pH 7 (2, 3). Although there is some variation among these proteins, the rank in terms of radiolability of amino acids is methionine, phenylalanine, histidine, cystine, serine, and threonine. However, whether this radiolability of amino acids is also true for other proteins and free amino acids was not ascertained. Our knowledge of the comparative radiolability of free amino acids in solutions is limited to results reported in a few studies. Dale et al. (4) compared the yield of ammonia from various irradiated amino acids with glycine as the reference. However, the extent of deamination of amino acids by radiation did not reveal a definite pattern of radiolability of amino acids. Barron (5) did not obtain ammonia from radiation of dilute solutions of glycine, and similar discrepancies have been observed by others (6, 7). Critical appraisal of observations, by different workers, of the effect of radiation on amino acids in solution is complicated because of several variables. Concentration, solubility and pH of amino acid solutions, presence or absence of buffer, oxygen tension of the medium, and quality and quantity of radiation dose employed often modify the effect of radiation. The purpose of this research was to determine the comparative radiolability of amino acids of proteins and free amino acids.