Non-enzymic nature of the pyridine haemochrome-cleaving activity of mammalian tissue extracts ('haem alpha-methenyl oxygenase').

1. The pyridine haemochrome-cleaving activity of extracts from mammalian liver and other tissues is shown conclusively to be entirely non-enzymic in nature and attributable to coupled oxidation with ascorbate. 2. Reduced glutathione probably contributes to the activity indirectly by continuously regenerating the ascorbate to the reduced form. 3. The cleavage shows no specificity for the alpha-methine bridge of pyridine haemochrome. 4. Results are presented suggesting some probable reasons for the erroneous characterization of the activity as an alpha-methine-specific haem-cleaving enzyme (;haem alpha-methenyl oxygenase') by Nakajima and co-workers (e.g. Nakajima, Takemura, Nakajima & Yamaoka, 1963; Nakajima & Gray, 1967).