Pyridoxal reaction with histidine

Further reactions may occur after the formation of a Schiff base. Thus aldehydes react with substituted phenylethylamines under mild experimental conditions to form ultimately tetrahydroisoquinolines i . A particular case of this is the reaction between pyridoxal and DOPA [Figure 4.5). A similar reaction occurs between pyridoxal and histidine [Figure 4.5). The observation that DOPA decarboxylase is subject to substrate inhibition aroused... 

The reaction of pyridoxal with a primary amine and the reduction of the Schiff base to the pyridoxyl derivative are shown in Figure 11. As only primary amino groups form Schiff bases, the reaction should be specific for compounds with primary amino groups. However, proline and hydroxyproline react as well and, as noted [2611 even secondary amines can be brought to reaction, esjjecially in non-aqueous solvents. A ketoenimine is assumed to be an intermediate of the reaction sequence in this case. Histidine reacts first via the primary amino group to form a Schiff base, which is rearranged to a non-redudble imidazolopyridine derivative. 

In addition, some essential food constituents react with formation of biologically inactive derivatives. Examples are riboflavin, pyridox-ine, niacin, folic acid, histidine or methionine. However, these reactions are not of importance under the conditions of the normal application of ethylene oxide or propylene oxide. 

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