The metabolism of collagen and other proteins of the skin of rabbits

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Abstract
Proteins in the skin of growing rabbits were separated into the following main fractions: (a) material soluble in phosphate buffer pH 9; (b) proteins insoluble in phosphate, but soluble in citrate pH 3.8; (c) collagen not extracted by these procedures, but con-verted into gelatin by autoclaving; (d) an insoluble residue. Evidence was obtained that the phosphate-soluble material was a mixture of cellular proteins, plasma proteins derived from the extracellular space and a small amount of a collagenous protein ("alkali-soluble collagen"). The citrate-soluble fraction consisted of collagen-like material which is called acid-soluble collagen ("procollagen" of Orekhovich). The amounts of hydroxy-proline, giycine and tyrosine in the 3 collagenous fractions were similar, although there were some significant differences. Evidence was obtained that gelatin from insoluble collagen is either heterogeneous with respect to amino-acid composition or contaminated with protein of non-collagen type. Experiments using a single dose of (alpha-C14)-giycine and isolating the glycine as the 2,4-dinitrophenyl derivative showed that the proteins of the phosphate-soluble fraction, other than alkali-soluble col-lagen, had turnover rates similar to those of plasma and liver proteins. The highest radioactivity of the alkali-soluble collagen was observed about 24 hours after administration of the labelled giycine and exceeded the maximum values found for other proteins of the phosphate fraction or plasma proteins. Thereafter it dropped rapidly to values below those of acid-soluble or insoluble collagen. The radioactivity of the giycine of the acid-soluble collagen by comparison with that of alkali-soluble collagen in-creased and decreased very slowly and the activity/time curve of insoluble collagen was almost flat. It is concluded that alkali-soluble collagen is almost certainly a true precursor of the other collagen fractions. The metabolic role of acid-soluble collagen is more uncertain and its activity/time curve suggests that it is not a necessary intermediate in the formation of all the insoluble collagen of the skin.