Pyridoxal kinase activity in blood cells

Abstract

The activity of the pyridoxal phosphate-forming enzyme, pyridoxal kinase, has been demonstrated in erythrocytes and leucocytes. The activity in erythrocytes was found to be 197 ± 18 · 10 −9 (M ± S.E.M) ng pyridoxal phosphate/h/cell (1.33 ± 0.12 · 10 −11 nmoles/min/cell) and in leucocytes between 29.7 ± 7.9 · 10 −6 (M ± S.E.M.) ng pyridoxal phosphate/h/cell (2.00 ± 0.53 · 10 −9 nmoles/min/cell). A K m value for pyridoxal of about 5 · 10 −6 M was found, both in erythrocytes and in leucocytes. The activity was dependent on the ATP concentration in the incubation mixture ∗. In erythrocytes this dependency was less pronounced than in leucocytes. A K m value for ATP of 3.1 · 10 −5 M was found in leucocytes. Also intact erythrocytes were shown to form pyridoxal phosphate from pyridoxal added outside the cell. A small amount of pyridoxal phosphate could be traced even outside the intact erythrocyte, indicating the passage of pyridoxal phosphate through the cell membrane. The erythrocytes were shown to form pyridoxal phosphate from pyridoxal and pyridoxol at about the same rate, while the formation from pyridoxamine was slower. The activity of pyridoxal kinase in erythrocytes was correlated with the pyridoxal phosphate concentration in plasma. Six patients with thyreotoxicosis were shown to deviate from this correlation in having increased activity of pyridoxal kinase with subnormal or normal pyridoxal phosphate concentrations in the plasma. This is interpreted as showing an increased synthesis of the kinase to compensate for the increased demand for pyridoxal phosphate in hyperthyreosis.