C onversion of Proline to Collagen Hydroxyproline in a Cell-free System from Chick Embryo* BEVERLY PETERKOFSKY AND SIDNEY UDENFRIEND

There is a unique relationship between proline hydroxylation and collagen biosynthesis, since proline, rather than free hydroxyproline, appears to be the major precursor of collagen hydroxyproline (1, 2). The early stages of collagen biosynthesis have been shown to be similar to the steps involved in the biosynthesis of other proteins in that hydroxyproline with the highest specific activity appears in microsomal protein when proline-‘% is introduced into systems in vitro or in vivo (3, 4). It has been possible to obtain a cell-free system from whole chick embryos which converts proline-‘% to hydroxyproline-rC found in a microsomal protein (5, 6). Although the microsomal protein is obviously not in the fibrous form usual for collagen, it has many of the physical and chemical properties of collagen, as will be shown in this report. For purposes of convenience and clarity, the term collagen will be used in referring to the hydroxyproline-containing microsomal protein. Some properties and requirements of the cell-free system, consisting of microsomal and supernatant fractions, are described, and studies on the mechanism and site of proline hydroxylation are reported.