Mammalian histidine decarboxylase: Activation of the apoenzyme with phospho-5′-pyridoximinotriazole

Histidine decarboxylase (apoenzyme) from a murine mastocytoma was activated by phospho-5′-pyridoximinotriazole (PPIT), a condensation product of pyridoxal-5′-phosphate (PALP) and 4-amino-1,2,4-triazole. PPIT was found to be less active at low concentrations and more active at higher concentrations than PALP in equimolar concentrations. Excess PPIT did not cause inhibition of the enzyme activity as did excess PALP. It was established that PPIT takes part in transaldimination reactions. It appeared that its coenzyme activity was a consequence of an initial transaldimination reaction in which PPIT reacted with a free amino group at the apoenzyme active site (i.e. the binding site for the natural coenzyme, PALP) leaving the conventional holoenzyme (the PALP-apoenzyme complex) and free aminotriazole. PPIT readily took part in transaldimination reactions with certain amino acids but not with others. It is possible that a systematic study of the selective readiness of various PALP derivatives with coenzyme activity to engage in transaldimination reactions may provide an analytical tool for identifying the amino acid which functions as the PALP binding site of the apoenzyme.