THE PENTOSE PHOSPHATE METABOLIC PATHWAY IN THE HUMAN ERYTHROCYTE: II. THE TRANSKETOLASE AND TRANSALDOLASE ACTIVITY OF THE HUMAN ERYTHROCYTE

Abstract

Transketolase, in the human erythrocyte, is confined to the soluble fraction of the cell. The activation energy for the formation of sedoheptulose-7-phosphate (S-7-P) from pentose phosphate was found to be 11,500 calories and the rate of formation of S-7-P to be directly proportional to the concentration of the enzyme. The Michaelis constant, with ribose-5-phosphate (R-5-P) as the added substrate, was found to be 6 × 10⁻³ M. The activity of the enzyme is close to the maximum over a wide range of hydrogen ion concentration (pH 7.1 to 8.3) with only a gradual decrease beyond these limits. The transketolase, in the dialyzed stroma-free hemolyzate, is active without the addition of magnesium ions or thiamine pyrophosphate. It is unaffected by sulphydryl-binding inhibitors and by EDTA and oxythiamine pyrophosphate.Transaldolase activity also has been demonstrated in the hemolyzate of human red cells. The rate of the production of hexose phosphate from sedoheptulose-7-phosphate was found to be of the order of 40 μmoles/g Hb/hour. The activity of the enzyme is close to the maximum between pH 7.18 and 7.75.