Crossroads of homocysteine, nitric oxide and asymmetric dimethylarginine metabolisms:involvement of S-adenosylhomocysteine and impaired cellular methylation

Tese de doutoramento, Farmácia (Bioquímica), Universidade de Lisboa, Faculdade de Farmácia, 2012 ; Elevated homocysteine, or hyperhomocysteinemia, is an independent risk factor for vascular disease. However, the precise mechanisms underlying this association, although intensively studied, are still incompletely solved. The precursor of all homocysteine produced in the body is S-adenosylhomocysteine (AdoHcy), a strong methyltransferase inhibitor. When homocysteine accumulates, AdoHcy accumulates as well, potentially disturbing most of the transmethylation processes within the cell. Notably, the role of AdoHcy has gained increased attention, regarding the pathophysiology of hyperhomocysteinemia. In fact, in recent studies, not homocysteine, but rather AdoHcy emerged as a more insightful indicator of vascular disease and tissue damage. Thus, in the present work, we postulate that elevated homocysteine itself may not be the major causative factor for the development of vascular disease, and sought to investigate whether AdoHcy accumulation, by disturbing cellular transmethylation reactions, would influence vascular homeostasis. Specifically, we will focus on the endothelium production of nitric oxide (NO), a potent anti-atherogenic molecule, and on the metabolism of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of the endothelial nitric oxide synthase (eNOS). Chapter 1 comprises a general introduction on homocysteine metabolism, including a review of the major concepts concerning regulation, determinants of hyperhomocysteinemia and proposed pathophysiological vascular mechanisms, with special interest on the NO/ADMA pathways. Chapter 2 presents the aims and outline of this thesis. We first investigated the impact of an impaired methylation environment due to AdoHcy accumulation on NO bioavailability. For that, we cultured human umbilical vein endothelial cells (HUVEC) with adenosine-2,3-dialdehyde (ADA), a well-known inhibitor of the enzyme that reversibly converts AdoHcy in homocysteine, to elevate ...