Oxygen fixation into hydroxyproline of plant cell wall protein.

The discovery of bound hydroxyproline in higher plants (1, 2) and the demonstration that this hydroxyproline is in peptide linkage (2, 3) and localized in the primary cell wall (3-8) shows that study of the metabolism and role of hydroxyproline can no longer be restricted to the field of collagen studies. Previous work (3-6) indicates that the wall-bound hydroxyproline is derived from free proline but not hydroxyproline, and exists in a protein which is soluble when first synthesized, but rapidly becomes firmly associated with the wall, possibly through a covalent cellulose-protein linkage. Of several problems raised by these findings, one under current study is the general problem of proline hydroxylation and its relation to cell wall metabolism and growth. Scharpenseel and Wolf (9) found very little incorporation of tritium into hydroxyproline when fetal rat skin was incubated in tritiated water. They also found that hydroxyproline synthesis was oxygen-dependent. They concluded that the hydroxylation of proline might involve fixation of atmospheric oxygen. This paper describes a single experiment designed to show whether or not the hydroxyproline hydroxyl oxygen in higher plants is derived via direct oxygen fixation. The experiment involved growth of sycamore (Acer pseudoplatunus L.) cell suspensions in a constant atmosphere of 0+N214 followed by isolation of pure hydroxyproline. Only one batch of cells was grown under the above conditions, but hydroxyproline was isolated from two cell fractions and then assayed for isotopic abundance. The paper concludes with discussion of the way in which a direct oxygenation reaction may be related to the control of cell extension growth mediated through changes in cell wall plasticity. While this work with plant cell suspension cultures was progressing, reports of direct oxygen fixation into hydroxyproline of collagen have appeared (10, 11).