A pathway for the metabolism of vitamin D ₃ : Unique hydroxylated metabolites formed during catalysis with cytochrome P450scc (CYP11A1)

Abstract

Metabolites of vitamin D3 (D3) (cholecalciferol) are recognized as enzymatically formed chemicals in humans that can influence a wide variety of reactions that regulate a large number of cellular functions. The metabolism of D3 has been extensively studied, and a role for three different mitochondrial cytochrome P450s (CYP24A, CYP27A, and CYP27B1) has been described that catalyze the formation of the 24(OH), 25(OH), and 1(OH) metabolites of D3, respectively. The hormone 1,25-dihydroxyvitamin D3 has been most extensively studied and is widely recognized as a regulator of calcium and phosphorous metabolism. Hydroxylated metabolites of D3 interact with the nuclear receptor and thereby influence growth, cellular differentiation, and proliferation. In this article, we describe in vitro experiments using purified mitochondrial cytochrome P450scc (CYP11A1) reconstituted with the iron-sulfer protein, adrenodoxin, and the flavoprotein, adrenodoxin reductase, and show the NADPH and time-dependent formation of two major metabolites of D3 (i.e., 20-hydroxyvitamin D3 and 20,22-dihydroxyvitamin D3) plus two unknown minor metabolites. In addition, we describe the metabolism of 7-dehydrocholesterol by CYP11A1 to a single product identified as 7-dehydropregnenolone. Although the physiological importance of these hydroxylated metabolites of D3 and their in vivo formation and mode of action remain to be determined, the rate with which they are formed by CYP11A1 in vitro suggests an important role.C ytochrome P450scc (CYP11A1) is a mitochondrial hemeprotein oxygenase of great interest because of its identification as a key enzyme in the metabolism of cholesterol to pregnenolone (1). This reaction is the initial step in the pathway leading to the synthesis of a number of metabolically important steroid hormones. Like other mitochondrial P450s this enzyme requires a minielectron transport chain consisting of the iron-sulfur protein, adrenodoxin (Adr), and the FADcontaining NADPH-Adr reductase (AdrR) for the transfer of reducing equivalents from NADPH to the P450. To date, CYP11A1 has no known natural substrates other then cholesterol. The conversion of cholesterol to pregnenolone (P5) is reported to occur by forming in sequence the monohydroxylated product [22R-(OH)cholesterol] followed by the formation of the dihydroxylated intermediate [20␣, 2 cholesterol] with subsequent carbon-carbon bond cleavage at the C20-C22 position to form the C19 steroid pregnenolone (1-5). It has been difficult to identify these hydroxylated intermediates of cholesterol metabolism during catalysis by CYP11A1 because they are not released from the active site of CYP11A1 during metabolism (1-5). Of interest is the recent report that the oxysterol 22R-(OH)cholesterol plays an important role as a ligand for the liver X receptor, a transcription factor (6).In mammalian tissues 7-dehydrocholesterol (7-DHC) is the direct precursor of cholesterol in the Kandutsch-Russell cholesterol biosynthetic pathway. A deficiency of ⌬7-DHC reductase, the enzyme responsible fo...

A Deficiência de vitamina d está associada com níveis aumentados de il-17 e tnfα em pacientes com insuficiência cardíaca crônica

“…However, these levels remained insufficient (data not shown). This finding supports several previous reports which suggested that statins may increase the synthesis of calcitriol by inducing enzyme 7-dechydrocholesterol 23,24 .…”

Structural basis for pregnenolone biosynthesis by the mitochondrial monooxygenase system

“…Contrary to drug-metabolizing but similarly to P450s involved in physiological functions, CYP11A1 has narrow substrate specificity-limited to cholesterol, 7-dehydrocholesterol, and vitamin D (6)(7)(8) (Fig. S1 and Table S1).…”

Effects of Atorvastatin on Vitamin D Levels in Patients With Acute Ischemic Heart Disease

“…Cholesterol is synthesized from 7-dehydrocholesterol, which is also a precursor of vitamin D 3 . For this reason, we initially observed a statistically significant relation between total cholesterol and vitamin D. HMG-CoA enzyme reductase is key to the synthesis of cholesterol, whereas ultraviolet radiation causes the formation of 25-hydroxyvitamin D. Inhibition of the enzyme may increase levels of 7-dehydrocholesterol and increase the synthesis of 25-hydroxycholecalciferol, thereby increasing vitamin D levels, 10 although we observed no relation between lower cholesterol and increased vitamin D. In addition, 25-hydroxyvitamin D has been shown to inhibit HMG-CoA enzyme reductase activity in in vitro studies. 11 A greater concentration of vitamin D could increase enzymatic inhibition, acting in synergy with the statin in decreasing total cholesterol.…”