Localization of the α-chain cross link acceptor sites of human fibrin

The potential cross-link acceptor sites of fibrin were specifically labeled with the fluorescent, substitute cross-link donor monodansyl cadaverine (MDC). Several fluorescent α-chain peptides generated from enzymatic and cyanogen bromide (CNBr) cleavage of the labeled fibrin were identified by sodium dodecyl sulfate disc gel electrophoresis; they were isolated and then characterized by amino acid analysis, NH₂-terminal sequence analysis, and chromatographic and electrophoretic analyses of their digestion products. Ancrod cleavage of MDC-labeled fibrin produced a series of six α-chain peptides of molecular weight 34,000 to 12,000, each of which contained an MDC-labeled acceptor site, and an NH₂-terminal α-chain derivative of molecular weight 37,500. The latter remains disulfide bound in the residual fibrin and has two MDC-labeled which are separable by CNBr cleavage. Mild plasmin digestion of MDC-labeled fibrin generated fluorescent α-chain peptides of molecular weights 45,000, 42,000, 35,000, 23,000, 21,000, and 2,500 in the supernatant and a nonfluorescent NH₂-terminal α-chain derivative of molecular weight 25,000 which remained in the insoluble residual fibrin. The alignment of these plasmic supernatant peptides was determined from NH₂-terminal sequence analyses which indicated that an MDC acceptor site was located at approximately residue 255 of the Aα-chain. Cleavage of the MDC-labeled α-chain by CNBr, however, localized most of its fluorescence (~80%) to a fragment of molecular weight 29,000 which had the same NH₂-terminal sequence as the labeled plasmic peptide of molecular weight 21,000. Both peptides were cleaved by ancrod into two acceptor site-containing peptides of approximately equal fluorescence. The preliminary NH₂-terminal sequence analyses of these peptides, when combined with the above findings, indicated that these two other cross-link acceptor sites are in a peptide segment which comprises the middle 17% of the Aα-chain.