Levels and subcellular localisation of pyridoxal and pyridoxal phosphate in human polymorphonuclear leukocytes and their relationship to alkaline phosphatase activity.

Neutrophil leukocytes, isolated from normal subjects and subjected to analytical subcellular fractionation by sucrose density gradient centrifugation, showed very similar cytosol distributions of pyridoxal and pyridoxal phosphate and lactate dehydrogenase. The small amounts of pyridoxal and pyridoxal phosphate associated with the dense granule fractions were not associated with the alkaline phosphatase containing granules. The levels of pyridoxal and pyridoxal phosphate were determined in neutrophils from control subjects, women in the third trimester of pregnancy and patients with chronic granulocytic leukaemia. Neutrophil pyridoxal phosphate was increased in women in the third trimester of pregnancy compared to controls, but there was little variation in the level of pyridoxal between the groups. There was no consistent correlation between the pyridoxal phosphate and the neutrophil alkaline phosphatase activity in the patient groups. Although in vitro neutrophil alkaline phosphatase rapidly hydrolyses pyridoxal phosphate, it is suggested that in vivo this is unlikely to be the principal function of the enzyme.