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

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 it‟s predominance in the absence of AdoCbl, with a consequent lowering of the . NO/ONOOratio, is the real source of supposedly . NO derived pathology.