A reexamination of the properties of spinach nitrite reductase: Protein and siroheme content heterogeneity in purified preparations

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Recent preparations of nitrite reductase do not display the heterodimeric quaternary structure obtained previously (total molecular weight 85,000; subunit molecular weights 24,000 and 61,000), but rather yield only the 61,000 molecular weight subunit, even when buffers containing the protease inhibitor phenylmethylsulfonyl fluoride are used. Nevertheless, such preparations retain the high ratio of ferredoxin-linked to methyl viologen-linked enzyme activity which has been previously taken as a characteristic of only the heterodimeric form. These preparations display a siroheme prosthetic group to protein ratio of 1.1. When nitrite reductase samples are frozen during the purification scheme, even though the ferredoxin-linked specific activity does not significantly decrease, enzyme activity-stained native gel electrophoresis of the subsequently purified protein reveals that gels with several bands of activity can be obtained. Further evidence of protein heterogeneity in these preparations comes from N-terminal amino acid analysis which reveals that even nonfrozen preparations contain two major peptides with valine and cysteine as the N-termini. Formation of complexes of purified nitrite reductase with ferredoxin resulted in siroheme difference electronic spectra which resembled those observed previously for monomeric preparations. However, the siroheme midpoint potential of recent preparations of nitrite reductase (−287 mV) is close to that of the heterodimeric preparations. Ultrafiltration studies of crude extracts of the enzyme indicate that, at least at certain stages of the preparation, higher molecular weight forms of the enzyme may exist. We conclude that the 24,000 molecular weight polypeptide is a contaminant and that the heterodimeric quaternary structure model for spinach nitrite reductase is incorrect. Furthermore, the monomeric preparations we do obtain display both significant protein heterogeneity and facile loss of siroheme upon gel filtration.