Homocysteine– A Stealth Mediator of Impaired Wound Healing: A Preliminary Study

4/1/2006
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Author(s):

Joseph V. Boykin, Jr., MD,1,2 and Chris Baylis, PhD3

Discussion

The results of this preliminary clinical study suggest a correlation exists between elevated serum Hcy and impaired chronic wound healing with patients receiving dermal substitute therapy. Furthermore, the preliminary data indicate that elevated serum Hcy may also be correlated with significantly decreased wound NO bioactivity as determined by wound fluid NOx assay, and left untreated, elevated Hcy may become common among patients with chronic wounds (50% incidence). Additionally, the observations of a single case report suggest that successful treatment of elevated Hcy in a patient with impaired wound healing may promote the restoration of normal wound healing.
Nitric oxide, a key mediator of cutaneous physiology, is formed by the enzymatic combination of molecular oxygen and the semi-essential amino acid L-arginine. Nitric oxide provides cellular signaling by activation of its target molecule, guanylate cyclase, which elevates intracellular concentrations of cyclic guanosine monophosphate (cGMP).²¹ Increased cGMP causes vascular smooth muscle relaxation, which constitutes a significant mechanism of homeostasis for microcirculation, and modulates the cardiovascular response to vasoconstrictors, cytokines, and endotoxin. Nitric oxide may alter key enzymes, affecting subcellular systems, the Krebs cycle, or RNA/DNA synthesis. This activity is performed without the need for signal transduction. Nitric oxide crosses cell membranes without mediation of channels or receptors—it diffuses across cellular membranes isotropically.²¹ Because of its high diffusion coefficient, short half-life (approximately 5 seconds), and prompt decomposition, NO is ideal because it acts as a cellular signal for wound repair. Nitric oxide is generated by 3 isoforms of nitric oxide synthase (NOS) that metabolize L-arginine and molecular oxygen to citrulline and NO.²² Two of the 3 isoforms are constitutive enzyme systems (cNOS) that are described in neuronal cells (nNOS) and endothelial cells (eNOS). With these enzymes, increased levels of intracellular calcium activate the cNOS via calmodulin. The calcium-dependent cNOS systems produce low (picomolar) quantities of NO. The third system is the inducible isoform (iNOS), which is calcium independent. Expression of iNOS is controlled by tissue-specific stimuli, such as inflammatory cytokines or exogenous materials, ie, bacterial lipopolysaccharide (LPS). Once induced, production of NO within tissue can increase as much as 1,000-fold, thereby producing an environment that is toxic to invading microorganisms. Currently, it appears that the cNOS enzymes are involved in maintaining skin homeostasis and providing regulatory function.²² The iNOS enzymes appear to be mainly associated with inflammatory and immune responses that are also implicated in certain skin diseases. In human skin, keratinocytes, fibroblasts, and endothelial cells possess both the cNOS and iNOS isoforms. The wound macrophage and keratinocyte possess the iNOS isoform.²³ Epithelial migration,⁴ wound angiogenesis,² and granulation tissue formation³ are primarily mediated by the activation and upregulation of the iNOS isoform.
The major metabolic pathway for NO is through nitrate (NO3–) and nitrite (NO2–), collectively termed NOx, which are stable metabolites within tissue, plasma, and urine.²¹ Tracer studies in humans have demonstrated that perhaps 50% of the total body NOx originates from the NO synthesis substrate, L-arginine, although this percentage will vary with the dietary intake of NOx.^24,25 Fasting plasma and urine samples allow clinicians to use variations in NOx values as a means of evaluating changes in NO production and bioactivity.²⁶

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