Hydroxyproline formation

The present work was undertaken in an attempt to shed light on the mechanism of hydroxyproline formation in collagen biosynthesis and on the site of action of ascorbate. It was recognized that these phenomena were very likely to be closely interrelated. Proline labeled with tritium was used because it was anticipated that tritium released from proline during hydroxylation would appear in the tissue water as tritiated water. The use of tritiated proline in such a system might then provide a new parameter for following the hydroxylation reaction, provided that the formation of tritiated water was stoichiometrically related to the formation of hydroxyproline. Such an approach might conceivably permit the demonstration of hydroxylation and peptide bond formation as separate chemical steps. 

Smith and Mitoma (43) have found an enzyme activity in mammalian tissues capable of reducing 4-keto-L-proline to L-hydroxyproline such a reaction (perhaps involving bound forms of ketoproline or hydroxyproline) might conceivably function in hydroxyproline formation. 

Deprivation of ascorbic add results in the inability to form collagen, as measured by hydroxyproline formation. Ascorbic acid given to scorbutic animals 10-12 days after wounding induced hydroxyproline for-... 

Little is known about the nature of this conversion. Presumably the major amount of hydroxyproline formation occurs when proline is bound in peptide linkage in collagen. This was suggested first by Stetten and Schoenheimer (133). The work of numerous subsequent investigators have supported this (133-136). Mitoma et al. (136) observed a minor degree of incorporation of hydroxyproline into protein in embryonic tissue, but this was overshadowed by the hydroxylation of the peptide-bound proline. 

The importance of a low rate of hydroxyproline formation in cell-wall... 

Floyd, R.A. Zs-Nagy (1984). Formation of long lived hydroxyl free radical adducts of proline and hydroxyproline in a Fenton reaction. Biochimica Bio-physica Acta, 790, 94-7. 

Vitamin C is essential for the formation of collagen, the principal structural protein in skin, bone, tendons, and ligaments, being a cofactor in the hydroxylation of the amino acids proline to 4-hydroxyproline, and of lysine to 5-hydroxylysine. These hydroxyamino acids account for up to 25% of the collagen structure. Vitamin C is also associated with some other hydroxylation reactions, e.g. the hydroxylation of tyrosine to dopa (dihydroxyphenylalanine) in the pathway to catecholamines (see Box 15.3). Deficiency leads to scurvy, a condition characterized by muscular pain, skin lesions, fragile blood vessels, bleeding gums, and tooth loss. Vitamin C also has valuable antioxidant properties (see Box 9.2), and these are exploited commercially in the food industries. 

Ascorbic acid or vitamin C is found in fruits, especially citrus fruits, and in fresh vegetables. Man is one of the few mammals unable to manufacture vitamin C in the liver. It is essential for the formation of collagen as it is a cofactor for the conversion of proline and lysine residues to hydroxyproline and hydroxylysine. It is also a cofactor for carnitine synthesis, for the conversion of folic acid to folinic acid and for the hydroxylation of dopamine to form norepinephrine. Being a lactone with two hydroxyl groups which can be oxidized to two keto groups forming dehydroascorbic acid, ascorbic acid is also an anti-oxidant. By reducing ferric iron to the ferrous state in the stomach, ascorbic acid promotes iron absorption. 

It is reported that GPA-1734 inhibits prolyl and lysyl hydroxylations by Fe chelation and that the formation of non-dialyzable labeled hydroxyproline was inhibited 70% at a concentration of 50 pM, yet incorporation of proline into total liver protein was unaffected at this concentration (78BBA(538)328). GPA-1734 is a potent inhibitor of basement membrane synthesis (78MI21002). 

Hydroxyproline and hydroxylysine Collagen contains hydroxy proline (hyp) and hydroxylysine (hyl), which are not present in most other proteins. These residues result from the hydroxylation of some of the proline and lysine residues after their incorporation into polypeptide chains (Figure 4.6). The hydroxylation is, thus, an example of posttranslational modification (see p. 440). Hydroxy proline is important in stabilizing the triple-helical structure of colla gen because it maximizes interchain hydrogen bond formation. 

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