METABOLISM OF RABBIT SKIN COLLAGEN - DIFFERENCES IN THE APPARENT TURNOVER RATES OF TYPE-I-COLLAGEN AND TYPE-III-COLLAGEN PRECURSORS DETERMINED BY CONSTANT INTRAVENOUS-INFUSION OF LABELED AMINO-ACIDS

Growing rabbits were infused for up to 10h with labelled proline, tyrosine and leucine to achieve plateau conditions within body free pools: for [3H]proline infusion, blood free-proline specific radioactivity remained constant after about 1h. For individual animals, type-I- and type-III-collagen precursors were isolated by precipitation with (NH4)2SO4 and DEAE-cellulose chromatography. Experiments where 3H- and 14C-labelled proline and tyrosine were infused concurrently for different periods of time showed that type I procollagen reached plateau specific radioactivity within 3h and 90% of the plateau value after 2h infusion, corresponding to a calculated apparent t( 1/2 ) of less than 26min. Plateau values for type I procollagen were taken as precursor amino acid pool specific radioactivities. The type-III-collagen-precursor fractions consistently showed lower rates of label incorporation and, by assuming that both type I and type III collagens are synthesized from the same amino acid pools, kinetic analysis revealed an apparent t( 1/2 ) for the isolated type-III-collagen precursors of 3.9h. For proline, there were large variations between animals in the ratio between the precursor pool for collagen synthesis and the skin homogenate free pool (0.31±0.13, mean±S.D.), so that collagen-synthesis rates based solely on total tissue free-pool values for proline are subject to large and inconsistent errors.