HISTIDINE DECARBOXYLASE OF LACTOBACILLUS 30A .3. COMPOSITION AND SUBUNIT STRUCTURE

Histidine decarboxylase of Lactobacillus 30a has a [formula omitted] of 9.2 S, a molecular weight of 190,000, and contains ten half-cystine residues per mole of native enzyme. In the presence of urea, ten sulfhydryl groups react with p-mercuribenzoate, 5,5′-dithiobis(2-nitrobenzoic acid), or iodoacetic acid, indicating that there are no disulfide bridges in the enzyme. Mercaptidation of half of these sulfhydryl groups inactivates the native enzyme. Upon complete carboxymethylation or treatment with sodium dodecyl sulfate, the decarboxylase dissociates into ten subunits of mol wt 19,000, which are not further dissociated in 5 M guanidine hydrochloride. Digestion with carboxypeptidase A removes five tyrosine residues from native histidine decarboxylase without inactivating the enzyme. Similar treatment of S-carboxymethylhistidine decarboxylase liberates ten tyrosine residues and on longer digestion alanine and leucine. The results are consistent with a C-terminal sequence -Leu-Ala-Tyr for each of the ten subunits. Hydrazinolysis also indicates that tyrosine is the sole C-terminal amino acid. After acid hydrolysis of dinitrophenyl enzyme, dinitrophenylserine was the only α- dinitrophenylamino acid recovered, indicating that serine is present as an N-terminal amino acid. The low recovery of this amino acid leaves open the possibility of other terminal groups. A simplified procedure for discontinuous polyacrylamide gel electrophoresis in the presence of urea was used to separate the peptides resulting from cyanogen bromide cleavage of the decarboxylase. Results of cyanogen bromide cleavage, carboxypeptidase digestion, and tryptic peptide mapping support the presence of ten subunits in this enzyme, and indicate that these are either identical or nearly so. © 1968, American Chemical Society. All rights reserved.