Uptake and Metabolism of Hydroxyproline in Endosperm Cells of Lolium multiflorum (Ryegrass)

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Abstract
Hydroxyproline is rapidly removed from the medium by suspension-cultured endosperm cells of ryegrass (Lolium multiflorum) in a process which exhibits Michaelis-Menten kinetics and is dependent on temperature and metabolic energy. Following uptake by the cells, hydroxyproline is rapidly converted to proline by a route which appears to be more direct than the pathways of hydroxyproline metabolism in mammalian and bacterial systems and which results in substantial conservation of the pyrrolidine ring. Four hours after providing the cells with 5.5 nM [3H]hydroxyproline, approximately 50% of the radioactivity is recovered as intracellular imino acids, intracellular protein and extracellular peptide material. The radioactivity is associated predominantly with proline and hydroxyproline residues. In common with other plant and animal systems, hydroxyproline is not incorporated directly into protein. However, radioactivity originating in extracellular hydroxyproline is detected eventually in peptide linkage, where it appears first as peptidyl proline. In the extracellular fraction which contains arabinogalactan-protein, peptidyl proline is hydroxylated to form peptidyl hydroxyproline and, after 4 h, approximately 18% of the original 3H label is found in this fraction.