Some applications of periodic acid to the study of the hydroxyamino-acids of protein hydrolysates

Abstract

Threonine on treatment with periodate at pH 7 yielded 70% acetaldehyde. Acetaldehyde and higher homologues, unlike formaldehyde, were removed from aqueous soln. by simple aeration; this provided the basis of a micro-method for the detn. of threonine and higher homologues in complete protein hydrolysates and other material of biol. interest. The detn. was not interfered with by serine, "hydroxylysine" or other known naturally occurring amino-acids. The aldehyde material so obtained from a number of proteins consisted almost entirely of acetaldehyde. There was no evidence of the occurrence of higher homologues of threonine in these proteins. To demonstrate this, a micro-method for the separation of acetaldehyde from its higher homologues was developed, which depends on "carrier" distillation using ether as a solvent. By this means as little as 5% of propaldehyde was detected in 1 mg. of acetaldehyde. The principles of this type of separation, and its applicability to micro-separations in other homologous series, were discussed. The explanation of an earlier report [Martin and Synge, 1940] of propaldehyde in the aldehyde material from wheat gluten lay in the dimorphism of acetaldehyde dinitrophenylhydra-zone.[long dash]In an appendix, Dr. BELL discussed the relevant X-ray data. The conditions for precipitation by dimedone of the CH2O set free from serine, etc., by periodic acid were studied. The presence of other amino-acids seriously lowered the yield obtained. The production of NH3 by periodate from such hydroxyamino-acids as serine, threonine, [beta]-hydroxyglutamic acid and hydroxylysine in the presence of K2CO3 [cf. Van Slyke et al. 1940] was shown by control expts. to be applicable to the estimation of the aggregate of these substances in complete protein hydrolysates. The results of threonine and aggregate hydroxyamino-acid estimations on a number of proteins were tabulated and discussed. Using the methods here developed, the hydroxyamino-acid fraction obtained from a wool hydrolysate by the acetylation-benzoylation procedure of Synge [1939] was investigated. This procedure gave low recoveries of threonine. Nearly 2% of the N of wool was isolated in the form of fairly pure serine showing optical activity. A preparation agreeing in properties with the "hydroxylysine" of Van Slyke et al. was obtained from the lysine fractions of gelatin and isinglass hydrolysates by the acetylation-benzoylation procedure. This material, like Van Slyke''s, yielded ammonia and formaldehyde on treatment with periodic acid.