The Very Large Gorilla Sitting in the Room? Adenosylcobalamin is the Missing Link: its Radical and Tetrahydrobiopterin are the Principal in vivo Catalysts for Mammalian Nitric Oxide Synthases.

Carmen Wheatley


Mammalian nitric oxide synthases (NOS) are a source of the universal second messenger, and pivotal biochemical molecule, nitric oxide (.NO). NOS are assumed to function catalytically in a haem-centred manner, by analogy with cytochrome P450. Yet, they differ significantly. Cobalamin, vitamin B12, is believed to function almost solely as an .NO scavenger and, latterly, as a direct, physiological inhibitor of the NOS. Yet, in pathology, associated to cobalamin deficiency, functional or otherwise, NOS over-produce superoxide, peroxynitrite (ONOO-), and other reactive nitrite species, rather than .NO (Figure 7). This paper offers a radical, new solution to the gaps and inconsistencies in the current understanding of the mechanism of haem-centred NOS catalysis, which also challenges the other existing paradigm of cobalamin as just an .NO mop. Examination of a wide diversity of NOS and cobalamin-dependent enzyme structure-function studies, as well as data from the .NO/cobalamin chemical, biochemical, immunological, genetic, and clinical literature, offers indications that cobalamin, specifically, in one of its active forms, adenosylcobalamin (AdoCbl), may have a third, eukaryotic coenzyme function as the principal cofactor of well-regulated NOS catalysis in vivo. The AdoCbl-centred NOS reaction is described in detail (Figure 5), and some existing evidence that, in vitro, without AdoCbl, NOS turnover activity is significantly slower than in in vivo AdoCbl-rich environments, is presented. AdoCbl, in conjunction with tetrahydrobiopterin, couples NOS oxygen binding/activation to L-arginine hydroxylation and .NO synthesis much more effectively than does haem, overcoming NOS spatial and redox problems, leading to productive catalysis, decreased radical formation/escape, with a consequent increased ratio of .NO to ONOO- , and prevention of pathology (Figures 5 & 7). In vivo, haem-centred NOS catalysis may, in fact, be the back-up NOS reaction, and its predominance in the absence of AdoCbl, with a consequent lowering of the .NO/ONOO- ratio, is the real source of supposedly .NO derived pathology




adenosylcobalamin; nitric oxide synthase catalysis; tetrahydrobiopterin; ZnS2N2 on/off switch; nitrosylcobalamin.



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[ Note: Italian and Spanish journal references have been read by the author in the original language published. Russian papers are translated for the author by Mark Pallant-Tripp.]


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