Hydroxyproline, collagen synthesis

The striking fall in the specific activity of the collagen hydroxyproline isolated from deficient granulomas observed in the present study (Table HI) suggests an alternative scheme of collagen synthesis. 

AA serves as an important cofactor for enzymes. Lack of AA in food causes scurvy in humans due to inefficient collagen synthesis, caused by the inactivation of the Fe(II)-activating prolyl hydroxylase and lysyl hydroxylase which catalyze the formation of hydroxyproline and hydroxylysine as essential components for collagens. Prolyl hydroxylases can also hydroxylate conserved prolyl residues in the alpha subunit of the hypoxia-inducible transcription factor, which signals for proteasomal degradation of the transcription factor . The proper action of these hydrolases requires dioxygen, thus they can act... 

Most signs of scurvy can be related to inadequate or abnormal collagen synthesis. Ascorbate enhances prolyl and lysyl hydroxylase activities (Chapter 25). Collagen formed in scorbutic patents is low in hydroxyproline and poorly cross-linked, resulting in skin lesions, bone fractures, and rupture of capillaries and other blood vessels. The absolute amount of collagen made in scorbutic animals may also decrease independently of the hydroxylation defect. The anemia of scurvy may result from a defect in iron absorption or folate metabolism. 

The almost unique occurrence of hydroxyproline as a significant constituent of collagen has provided a convenient chemical measure of collagen synthesis. 

Prolylhydroxylase, the collagen synthesis enzyme that has low activity in silicon-deficient tissue, requires iron that cycles between Fe " " and Fe " ". The apoenzyme will bind aluminum (less strongly than iron), and the enzyme is then, of course, inactive. An experiment was conducted [6] in which the apoenzyme was presented with iron first and then aluminum, aU other essential cofactors being present Activity, as measured by hydroxyproline production, was reduced by 20% of the control level. When the apoenzyme was presented with aluminum first and then iron, activity was reduced by 55% of the control levels. Silicic acid alone had no effect on the activity, which remained at the control level (Table 58.1). 

There was another interesting development during this tRNA controversy. One of the proponents of hydroxylation of tRNA-Pro reached the following conclusions in a paper These findings do not, however, justify the statement of Peterkofsky and Udenfriend that their results rule out s-RNA hydroxyproline as an intermediate in collagen synthesis. The fact that such an intermediate has now been demonstrated by three groups of workers firmly establishes the existence of such an intermediate . Fortunately science does not advance by public opinion polls. Shortly thereafter one of the advocates of the tRNA site for hydroxylation carried out one of the key experiments that finally excluded this mechanism in the biosynthesis of hydroxyproline. 

A requirement for ascorbic acid in collagen formation has been well documented. This, it has been suggested, results from a utilization of this vitamin in the formation of hydroxjrproline from bound proline in a precollagen peptide (137,138) or for formation of an activated hydroxyproline (139). Mitoma and Smith (140) showed, however, that while ascorbic acid deficiency impaired collagen synthesis hydroxylation of proline was not affected. 

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. 

Inouye, K., Kobayashi, Y., Kyogoku, Y., Kishida, Y., Sakakibara, S., and Prockop, D. J. (1982). Synthesis and physical properties of (Hydroxyproline-Proline-Glycine)10 Hydroxyproline in the X-position decreases the melting temperature of the collagen triple helix. Arch. Biochem. Biophys. 219, 198-203. 

Sakikabara, S., Inouye, K., Shudo, K., Kishida, Y., Kobayashi, Y., andProckop, D.J. (1973). Synthesis of (Pro-Hyp-Gly) n of defined molecular weights. Evidence for stabilization of collagen triple-helix by hydroxyproline. Biockim. Biphys. Acta 303, 198-202. 

Figure 3.4 Factors affecting foreign body reaction and potential points of intervention at the level of the myofibroblast (1) inhibit synthesis or release of TGF-P (2) block stimulation by TGF-P of its membrane receptors on the activated fibroblast (3) inhibit the Smad proteins, which transfer the TGF-P effect to the nucleus (4) inhibit transcription of procollagen mRNA (5) inhibit translation of the message to form procollagen (6) inhibit prolyl-4-hydroxylase, which creates hydroxyproline and facilitates helix formation (7) inhibit lysyl oxidase, which cross-links the collagen (8) enhance the function of MMPs, which degrade collagen, or inhibit TIMPs, which degrade MMPs.

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