Pyridoxyllysine

In addition to the above reagents, which modify specific tyrosine residues in the protein, desensitization has been reported with pyridoxal phosphate, which forms a Schiff base derivative with lysyl residues (43). This reagent was first reported by Marcus and Hubert (43) to react with FDPase from swine kidney and to abolish AMP sensitivity with very little loss of catalytic activity. With liver FDPase most of the sensitivity to AMP is lost when 7-8 residues are incorporated, with concomitant loss of about 25% of the enzymic activity (43). The effects become irreversible when the Schiff base derivative is reduced with NaBH4 and Are-pyridoxyllysine has been isolated from the reduced complex. In the presence of AMP the sensitive lysine residues are protected, but the amount of PLP incorporated is increased (43). 

A proportion of the vitamin Be in foods may be biologically unavailable after heating, as a result of the formation of (phospho)pyridoxyllysine by reduction of the alditnine (Schiff base) by which pyridoxal and the phosphate are bound to the e-amino groups of lysine residues in proteins. A proportion of this pyridoxyUysine may be useable, because it is a substrate for pyridoxamine phosphate oxidase to form pyridoxal and pyridoxal phosphate. However, it is also a vitamin Be antimetaboUte, and even at relatively low concentrations can accelerate the development of deficiency in experimental animals maintained on vitamin Be-deficient diets (Gregory, 1980a, 1980b). 

Gregory JF 3rd (1980a) Effects of epsilon-pyridoxyllysine and related compounds onliver and brain pyridoxal kinase and liver pyridoxamine (pyridoxine) 5 -phosphate oxidase. Journal of Biological Chemistry 255, 2355-9. 

Gregory JF 3rd (1980b) Effects of epsilon-pyridoxyllysine bound to dietary protein on the vitamin B-6 status of rats. Journal of Nutrition 110, 995-1005. 

Colman and Frieden (108) demonstrated in 1966 that acetylation of one amino group per subunit with acetic anhydride produces 80% inactivation. More extensive acetylation alters the degree of polymerization and certain kinetic parameters (276). Almost simultaneously, Anderson et al. (277) reported the reversible inhibition of GDH by pyridoxal 5 -phosphate and certain other aromatic aldehydes. The inhibition was attributed to formation of a Schilf base since reduction with NaBH4 results in irreversible inactivation. It was estimated that approximately one residue of -pyridoxyllysine had been formed per subunit. In 1969, Holbrook and Jeckel (278) inactivated the enzyme by reaction with a substituted maleimide and, subsequently, obtained the partial sequence of a tryptic peptide containing a modified lysine residue (Fig. 7). 

Piszkiewicz et al. (279) isolated a peptide containing e-pyridoxyllysine from a tryptic digest of GDH which had been inactivated by the procedure of Anderson et al. (277). The sequence of the peptide demonstrates that the substituted residue is Lys-126 both in the bovine (279) and, later, in the chicken enzymes (133,280). 

Much of our knowledge of human vitamin Bg deficiency is derived from an outbreak in the early 1950s, which resulted from an infant milk preparation which had undergone severe heating in manufacture. The probable result of this was the formation of pyridoxyllysine by reaction between pyridoxal phosphate and the e-amino groups of lysine in proteins. In addition to a number of metabolic abnormalities, many of the affected infants convulsed. They responded to the administration of vitamin Bg supplements. 

---