Release of amino acids from soil humic acids by proteolytic enzymes

Abstract

Direct evidence for the occurrence of peptide bonds in soil humic acids has been obtained by showing that the proteolytic enzyme pronase releases a-amino acids from each of 10 humic acids. The maximal amounts of amino acids liberated by pronase accounted for 27.6-39.4% of those released by hot acid hydrolysis. The susceptibility of humic acids to proteolytic attack showed an approximate inverse relationship with their aromatic content, based on their absorbance at 260 or 450 mµ. Both pronase and papain attacked casein less readily when it was incorporated into a phenolic polymer prepared from p-benzoquinone. Papain was inactive towards soil humic acids. Similar amino acids were released by pronase from each of five humic acids tested. The amino acid patterns showed no basic amino acids but relatively high proportions of leucine (isoleucine) as compared with those obtained by acid hydrolysis. The action of pronase on humic acids had little effect on the molecular weight distribution of the humic components.