Pyridoxal hemiacetal form

This enzyme [EC 1.1.1.107] catalyzes the reaction of pyridoxal with NAD+ to produce 4-pyridoxolactone and NADH. The enzyme acts on the hemiacetal form of the substrate. 

Another system in which ring-formation has been considered to be manifested on polarographic curves is the reduction of pyridoxal (77, 80). The reduction wave of this compound changes with pH and the observed plot is similar to that shown in Fig. 22. This dependence can be explained either by hydration (as for other pyridine carboxaldehydes), or by hemiacetal formation. The same two interpretations can be applied to electronic spectra. A comparison with the behaviour of pyridoxal-5-phosphate can contribute to the solution of this problem. With this ester the formation of the hemiacetal form is impossible and practically no current decrease in acidic solutions can be observed. Hence it can be concluded that the decrease in the limiting current of pyridoxal is due to ring formation. Nevertheless, the possibility of some participation by a dehydration reaction cannot be completely excluded, for it is possible to assume that the introduction of a phosphoric acid residue into position 5 either shifts the equilibrium towards the dehydrated form or increases the rate of dehydration. 

Release of free vitamin, mainly pyridoxal, occurs when physiological nonsaturating levels of vitamin are absorbed. Here the phosphates are hydrolyzed by nonspecific alkaline phosphatase located on the plasma membrane of cells. Some PLP is also released into the circulation by the liver. Because the reactive aldehyde is capable of forming Schiff bases with amino groups, PLP in plasma is more tightly com-plexed to proteins—mostly albumin—than is pyridoxal, which forms an intramolecular hemiacetal between the... 

Likewise, the aldehydic and not the hemiacetal form of pyri-doxal undergoes polarographic reduction. Because the ringopening reaction of the hemiacetal group is acid-base catalysed, an increase of the limiting current at low and high pH-values was observed (Fig. 10). The acid-base catalysis can probably be explained by the difference in reactivity of different protonized forms of pyridoxal. In agreement with the explanation of the... 

It should be noted that PL can exist in solution as a cyclic hemlacetal, aldehyde hydrate, or free aldehyde. The cyclic hemiacetal form is predominantly found in solution (69) the fluorescence characteristics of PL listed in Table 2 therefore refer to the cyclic hemiacetal form of the vitamer. If pyridoxal occurred in the aldehyde form, the fluorescence observed should be considerably less, since the aldehyde group tends to diminish the fluorescence of aromatic systems due to its tendency to withdraw electrons from the ring structure (68,70). 

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