Enzyme proline oxidase

Figure 8.6 The three dehydrogenase (oxidase) reactions in amino acid degradation. The enzymes are D-amino acid oxidase, glutamate dehydrogenase and proline oxidase (dehydrogenase). Biochemical details are given in Appendix 8.4.

Figure 20.19 Biosynthesis and degradation of proline, hydroxyproline and ornithine. Proline oxidase and S-pyrroIine-5-carboxylic acid dehydrogenase are both mitochondrial enzymes. A and B indicate defects in hyperprolinemia I and II, respectively.

Proline racemase is a member of a broad family of cofactor-independent epimerases and racemases, and has been very well characterized mechanistically. The proline racemase from Clostridium sticklandii was the first of the cofactor-independent racemases to be characterized [13, 80], The enzyme participates in the catabolism of L-proline, producing o-proline as a substrate for o-proline oxidase [4]. Early... 

An inborn error of metabolism in which elevated hydrox] proline levels are found in the blood due to a deficiency in ti enzyme, hydroxyproline oxidase. Mental subnormality is feature. 

Increased blood levels are found in the rare inborn errors, the hyperprolinaemias. The metabolic defect in type I hyper-prolinaemia is an absence of proline oxidase, the first enzyme of the proline degradative pathway. A deficiency of A -pyrroline-5-carboxylic acid dehydrogenase, another enzyme involved in proline degradation, may be responsible for type II hyper-prolinaemia. 

More recently we have studied the effects of j3-thia-proline on protein synthesizing systems(56). -Thiapro-line,first synthesized by Eourneau et al. (57) ilia-S not been assayed in biological systems. We have obtained preliminary data indicating that it is a substrate for hog kidney D-aminoacid oxidase and for rat liver mitochondria proline oxidase,two enzymes active on y-thiaproline. 

Fig. 2. Metabolic scheme for the interconversions of proline, ornithine and glutamate. Enzymes catalyzing the reactions are designated by the following numbers (1) P5C synthase (2) P5C dehydrogenase (3) ornithine aminotransferase (4) P5C reductase (5) proline oxidase, Step 6 is spontaneous. Adapted from ref. (85).

Because the proposed transfers of redox mediated by the metabolism of P5C can be catalyzed by several enzyme mechanisms, inborn deficiencies of any one of the mechanisms would not necessarily result in a total absence of P5C-mediated redox transfers. In fact, pathophysiologic manifestations may be limited to those tissues in which the deficient mechanism plays a major physiologic role. For example, in gyrate atrophy of the choroid and retina with absent ornithine aminotransferase, the pathology is restricted to ocular tissues. Within the proposed scheme for P5C-mediated redox transfers, this tissue specificity may be due to the absence or relative deficiency of proline oxidase and P5C synthase in ocular tissues (61). The proposed redox transfer mechanisms would then be deficient since there is no source of P5C. We can speculate that the deficiency in the transfer of cytosolic NADPH (Table H) is related to the pathogenesis of the ocular pathology. 

Finally, the deficiency of proline oxidase in Type I hyperprolinemia has not been associated with any clinical manifestations (85). Although the defect was established in one subject with postmortem enzyme measurements (25), the diagnosis in living subjects is made by exclusion. However, PRO/Re mice, an inbred strain with hyperprolinemia is clearly proline oxidase-deficient (9). The behavioral aberrations and infertility in these mice have yet to be related to the deficiency in proline oxidase. 

The a-ketoacid-dependent enzymes are distinguished from other non-haem iron enzymes by their absolute requirement for an a-ketoacid cofactor as well as Fe(II) and O2 for activity. They catalyse two types of reaction (Table 2.3), hydroxyla-tion and oxidation. In both, the a-ketoglutarate is decarboxylated and one oxygen atom introduced into the succinate formed in the hydroxylases, the other oxygen atom is introduced into the substrate, while in the oxidases it is found in water, together with the cyclized product. In general these enzymes require one equivalent of Fe(II) an a-ketoacid, usually a-ketoglutarate and ascorbate. Examples of these enzymes include proline 4-hydroxylase, prolyl and lysyl hydroxylase, which... 

Reduction of D-proline by D-amino acid oxidase at pH 8 shows two steps when monitored at 640 nm. These are interpreted as the build-up and breakdown of a reduced enzyme-imino acid charge transfer complex. If the reaction is monitored using phenol red the same two rates are observed but additionally the release of = 1 proton for each step can be assessed and interpreted. The indicator changes are followed at 505 nm and 385 nm, which are isosbestic wavelengths for the two steps (without indicator),... 

Whereas some enzymes are highly specific to a single optical isomer (glucose oxidase, for example), other enzymes may catalyze reactions for several related substances. Thus D-amino acid oxidase reacts with several D-amino acid substrates. The relative rates are tyrosine, 100 proline, 78 methionine, 42 alanine, 34 serine, 22 tryptophan, 19.5 valine, 18.4 phenylalanine, 13.7 isoleucine, 11.6 leucine, 7.4 and histidine, 3.3. 

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