Two forms of NADH-cytochrome b5 reductase (b5R), an erythrocyte-restricted soluble form, active in methemoglobin reduction, and a ubiquitous membrane-associated form involved in lipid metabolism, are produced from one gene. In the rat, the two forms are generated from alternative transcripts differing in the first exon, however, biogenesis of human b5R was less understood. Recently, two different transcripts (M and S), differing in the first exon were also described in humans. Here, we have investigated the tissue-specificity and the role of the S-transcript in the generation of soluble b5R. By RNase protection assays designed to simultaneously detect alternative b5R transcripts in the same sample, the S transcript was undetectable in nonerythroid and in erythroleukemic K562 cells induced to differentiate, but was present in terminal erythroblast cultures, and represented a major b5R transcript in reticulocytes. Analysis of the translation products of the M- and S-transcripts in HeLa cells transfected with the corresponding cDNAs demonstrated that the S-transcript generates soluble b5R, presumably from an internal initiation codon. Our results indicate that the S-transcript is expressed at late stages of erythroid maturation to generate soluble b5R.
An in-frame deletion of codon 298 of the NADH-cytochrome b5 reductase gene results in hereditary methemoglobinemia type II (generalized type). A functional implication for the role of the COOH-terminal region of the enzyme.
