Subject 4-pyridoxic acid

Although, owing to the wide distribution of vitamin Bg in nature, clinical deficiency symptoms are seldom observed, there is little doubt that pyridoxine is essential in human nutrition. Pyridoxine is absorbed from the gastrointestinal tract and is converted to the active form pyri-doxal phosphate. Absorption is decreased in gastrointestinal diseases and also in subjects taking isoniazid (3). It is excreted in the urine as 4-pyridoxic acid (2). The metabolism of vitamin Bg in human beings has been investigated (56). 

Free pyridoxal either leaves the cells or is oxidized to 4-pyridoxic acid by aldehyde dehydrogenase (which is present in all tissues) and also by hepatic and renal aldehyde oxidases. 4-Pyridoxic acid is actively secreted by the renal tubules, so measurement of the plasma concentration provides an index of renal function (Coburn et al., 2002). There is some evidence that oxidation to 4-pyridoxic acid increases with increasing age in elderly people, the plasma concentration of pyridoxal phosphate is lower, and that of 4-pyridoxic acid higher, than in younger subjects even when there is no evidence of impaired renal function (Bates et al., 1999b). Small amounts of pyridoxal and pyridox-amine are also excreted in the urine, although much of the active vitamin Be that is filtered in the glomerulus is reabsorbed in the kidney tubules. 

Gershoff and Prien (G6) found that normal subjects excrete significantly less xanthurenic acid and 4-pyridoxic acid and more citric acid than patients with chronic formation of calcium oxalate. A marked rise in excretion of calcium oxalate followed administration of tryptophan in these patients, whereas ingestion of pyridoxine was followed by a decrease in urinary oxalate. 

Most of the vitamin in the body is eventually degraded to pyridoxic acid (PX) and excreted in the urine. Vitamin deficiency can result in a decrease in the amount of PX excreted, as illustrated by the data tn Table 9.4. Human subjects who had consumed a B i-sufficient diet were fed a B(,-deficient diet for 45 days. The results... 

The catabolic pathway of vitamin Bg (1) is probably the best studied. In animals (including humans) 4-pyridoxic acid (4) is the primary catabolic product of vitamin Bg, found in urine. It is formed by the oxidation of pyridoxal (2) by a nonspecific flavin adenine dinucleotide (FAD)-dependent aldehyde oxidase. The catalytically active form of vitamin Bg, pyridoxal-5 -phosphate, PLP (17), does not undergo similar oxidation to form 4. The various forms of vitamin Bg (1, 2, 15) and their respective phosphate esters (16, 17) are readily enzymatically interconvertible. Further degradation of 4 is unlikely in humans as the subjects administered with doses of 4 were found to excrete it quantitatively. Estimation of 4 in urine serves as a nutritional marker, as lower than normal amounts of 4 is indicative of vitamin Bg deficiency. ... 

JC Rabinowitz, EE Snell. Vitamin B group. XV. Urinary excretion of pyridoxal, pyridoxamine, pyridoxine, and 4-pyridoxic acid in human subjects. Proc Soc Exp Biol Med 70 235-240, 1949. 

Urinary excretions of nicotinic acid metabolites and 2-pyridone, as well as of 4-pyridoxic and xanthurenic acids were determined in 15 South African Bantu pellagrins before and after tryptophan administration (P13). Red blood cell riboflavine levels and serum glutamic-oxalacetic transaminase levels were also measured. The authors discussed the apparent inability of the pellagra patients to convert tryptophan to nicotinic acid as indicated by their low excretion of nicotinic acid metabolites before and after tryptophan load. The possibility that the subjects were also suffering from a riboflavine deficiency was also discussed. 

Figure 11.13 Reactions at a-carbon of a-amino acids catalyzed by pyridoxal enzymes All three substituents at C are subject to labilization in the three types of a-carbon reactions. The hydrogen is labilized in recemization reactions, the amino group is labUized in the transamination and the carboxyl group is labilized in decarboxylation. a-Amino acid condenses with pyridoxal phosphate to yield pyridoxylidene imino acid (an aldimine). The common intermediate, aldimine and distinct ketimines leading to the production of oxo-acid (in transamination), amino acid (in racemization) and amine (in decarboxylation) are shown. The catalytic acid (H-A-) and base (-B ) are symbolic both can be from the same residue such as Lys258 in aspartate aminotransferase.

The reactions of amino acids, catalysed by enzymes requiring a derivative of vitamin Bg (Figure 5a) as cofactor, have been the subject of a number of mechanistic studies. Enzymes catalysing decarboxylation, racemisation, dehydration (of serine or threonine) or desulphy-dration (of cysteine) require pyridoxal-5 -phosphate as cofactor (Figure 5b), and are dealt with in later sections. Enzymes catalysing transamination, on the other hand, require either pyridoxal-5 -phosphate or pyridoxamine-5 -phosphate (Figure 5c). Snell and coworkers showed that the reactions of amino acids normally... 

To better understand the mode of action of pyridoxal-P, let us examine in detail equation 7-3 the conversion of homosereine phosphate to threonine. It is an elimination-hydration transformation. The first process (Fig. 7.10) involves an aldimine ketimime tautomerization which is subjected to general-acid catalysis intramolecularly by the hydroxyl group, followed by the slow breaking of a C—H bond. The latter is the rate-determining step. 

Pyridoxal phosphate was demonstrated to be required as a coenzyme by dissociating it from the apoenzyme. This causes a loss of enzyme activity which can be restored by recombining the apoenzyme with pyridoxal phosphate. The optimum activity is at pH 8.3 and the enzyme requires free sulfhydryl groups for activity. The serine deaminase activity of the enz3rme is strongly inhibited by homocysteine and by cystathionine. No synthetic product appears to be formed with cysteine. The Km of the enzyme for serine is 8.1 X 10 Af. Knowledge of the mechanisms of the reactions catalyzed by pyridoxal phosphate-dependent enzymes has been advanced by model experiments with pyridoxal, metal ions, and amino acids. This subject has been reviewed recently by Snell (61). 

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