The localization of the extension aminopropeptides of Type III procollagen was studied in baboon liver by the immunoperoxidase method. The aminopropeptides were localized along the collagen fibrils around terminal hepatic venules, in the space of Disse, and in portal tracts of control baboons as well as of alcohol-fed animals. The labeling showed a characteristic periodicity of 60-70 nm, corresponding to the D (67 nm) stagger of collagen fibrils. The data indicate that some extension aminopropeptides of Type III procollagen are not cleaved from procollagen prior to its incorporation into collagen fibrils but that they exist as constituents of mature fibrils.
