Vitamin conservation effect

Katunuma and coworkers (1971) described a protease in the rat that hydrolyzes the apoenzymes of a number of pyridoxal phosphate-dependent enzymes it has no effect on other proteins or the holoenzymes. Presumably, it attacks the conserved amino acid sequence around the active lysine residue to which the internal Schiff base is formed. The activity ofthe enzyme is increased some 10- to 20-fold in vitamin Be deficiency, suggesting that its function is to degrade those enzymes that lose their coenzyme more readily, and so make more pyridoxal phosphate available for use by other enzymes. There is also evidence that some pyridoxal phosphate-dependent apoenzymes are modified to become incapable of activation by pyridoxal phosphate, although retaining immunological cross-reactivity with the normal form of the enzyme in vitamin Be deficiency (Nagata and Okada, 1985). 

A third consequence is the question of whether glycogen phosphorylase conserves vitamin B6 because it has a very low rate of degradation or because it recycles the pyridoxal phosphate very efficiently. Based on the rapid decay of the free vitamin B6 pool in muscle, the slow turnover of the protein-bound vitamin B6 pool, the failure to detect apo-phosphorylase, and the agreement between turnover rates based on amino acid or vitamin B6 data, Beynon et al. (1986) concluded that recycling would be minimal. We feel that the effect of vitamin B6 intake on turnover rates tends to favor the recycling option under conditions of limited intake. However, more data are needed before a final decision can be made. 

The conservation and flushing effects observed in vitamin B6 metabolism can be simulated by binding relationships. 

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