The Role of Cytochrome P450 Holoenzyme Metabolism in the Origin of Neuropathologies

The increased activity of δ-aminolevulinic acid synthase-1 (ALAS1) leads to the accumulation of δ-aminolevulinic acid (ALA), which may reduce the iron(III) retention ability of ferritin, resulting in iron(III) overload. The accumulation of ALA occurs in the condition of attenuation of the heme negative feedback loop over ALAS1 activity in concert with the induction of ALAS1 expression. Attenuation of the heme negative feedback loop is maintained by elevated quantities of the heme catabolizing enzyme, heme oxygenase-1 (HO-1), which, in turn, may be induced by highly increased heme concentration. The upregulation of brain HO-1 occurs in patients with Alzheimer's and Parkinson's diseases. A mouse with overexpressed human HMOX1 is a model of schizophrenia with concurrent iron overload. HO-1 inhibitors reduce oxidative damage to whole cells and mitochondrial compartments of rat astrocytes transfected with the HMOX1 gene. Iron reduction within the heme prosthetic moiety of P450 cytochromes in microsomes is facilitated by NADPH-cytochrome P450 oxidoreductase (CPR), while adrenodoxin reductase performs this function in the mitochondria. In partial CPR-deficient conditions, the half-life of apocytochromes is prolonged while HO-1 is induced due to the elevated heme release from unreduced cytochromes. This study posits that, before being degraded by HO-1, the released hemin triggers the oligomerization of cytochromes, as well as other oxidative damages, by producing hydroperoxyl radicals from hydrogen peroxide produced in uncoupling reaction. After reaching a specific level, iron(III) overload may trigger the saturation of CPR, resulting in the development of neuropathologies.