Participation of Cytochrome b5 in CMP-N- Acetylneuraminic Acid Hydroxylation in Mouse Liver Cytosol1

The activity of CMP-N-acetylneuraminic acid hydroxylase, that converts CMP-N-acetylneuraminic acid (CMP-NeuAc) to CPM-N-glycolylneuraminic acid (CMP-NeuGc), in mouse liver was determined by a newly developed HPLC method using non-radioactive CMP-NeuAc as a substrate. The activity was detected in the cytosol fraction but not in the microsomal fraction. Either NADH or NADPH was used as an electron donor by the cytosol enzyme, but NADH was much more efficiently used than NADPH. An antibody against cytochrome b5 markedly reduced the CMP-NeuAc hydroxylase activity when added to incubation mixture containing either NADH or NADPH as an electron donor. These data led us to postulate the following electron transport system, which is involved in the CMP-NeuAc hydroxylation in mouse liver cytosol: (formula; see text) where X, Y, and Z are components supposedly involved.