Mitochondrial P450s

Abstract

Cytochrome P450 was first found in the microsomes from animal tissues, and then the presence of P450 in mitochondria was reported for the steroidogenic organs, adrenal gland and gonads. Three forms of mitochondrial P450 (11A, 11B1, and 11B2) were purified from these organs and their functions in steroid hormone biosynthesis were confirmed. Later studies showed the presence of several other forms of P450 (24A, 27A, 27B, and 27C) in the mitochondria of various non-steroidogenic organs including liver and kidney. These mitochondrial P450s were found to participate in the biosynthesis of bile acids from cholesterol in the liver, and the metabolic activation of Vitamin D3 to its active form, 1,25-dihydroxyvitamin D3, in the liver and the kidney. In contrast to the “drug-metabolizing” P450s in microsomes, most mitochondrial P450s show high specificity to their endogenous substrates, and have negligible activity towards xenobiotic compounds. In contrast to these established roles of mitochondrial P450s in the metabolism of endogenous substrates, the metabolism of xenobiotic chemicals by P450-catalyzed reactions in mitochondria has long been a subject of controversy. It is now known that all P450s in eukaryotic organisms are coded by nuclear genes, and the nascent peptides of various forms of P450 synthesized by cytoplasmic ribosomes are targeted to either endoplasmic reticulum (ER) or mitochondria depending on the ER-targeting sequence or the mitochondria-targeting sequence present in their amino-terminal portion. However, the presence of some microsome-type P450s in the mitochondria from various animal tissues including liver and brain has been reported. Possible mechanisms of intracellular sorting of some microsome-type P450s to mitochondria have been proposed, although physiological significance of the contribution of P450s in mitochondria to the metabolism of xenobiotic chemicals in animal tissues is still elusive.

Introduction

The presence of cytochrome P450 in mitochondria was first found in adrenal cortex in 1964 [1], and then in other steroidogenic organs of mammals in the following years [2]. Three different forms of P450 (11A, 11B1, and 11B2) were purified from the mitochondria of adrenal cortex, gonads and ovary, and their roles in the biosynthesis of adrenal cortex hormones and sex steroid hormones from cholesterol was confirmed [3]. Participation of P450s in the biosynthesis of bile acids from cholesterol in the liver was then found [4], and a novel mitochondrial P450 (27A) was purified from liver and characterized [5]. It was also found that the metabolic conversion of Vitamin D3 to its active form, 1,25-dihydroxyvitamin D3, was catalyzed by two mitochondrial P450s, P450 27A in the liver [6] and P450 27B in the kidney [7]. Another kidney mitochondrial P450 (24A) catalyzes the 24-hydroxylation of 25-hydroxyvitamin D3, and regulate the level of the active form of Vitamin D3 [8]. All of these findings suggested that the P450s in the mitochondria in mammalian cells are distinct from their microsomal counterparts, and are specialized in the biosynthesis and metabolism of endogenous steroids and related compounds including Vitamin D3. P450s have been found in the mitochondria of diverse animal species including various vertebrates and insects, and their major functions are also metabolism of steroids and related endogenous substrates. Both microsomal and mitochondrial P450s are coded to nuclear genes, and the localization of specific forms of P450 to mitochondria depends on the mitochondria-targeting sequence in the amino-terminal portion of the nascent peptides, whereas microsomal P450s are sorted to endoplasmic reticulum (ER) by the ER-targeting sequence at the amino-terminus of the peptides.

Mitochondrial P450s are integral membrane proteins bound to the inner membrane, and receive electrons for monooxygenation reactions from NADPH via two soluble proteins in the matrix, adrenodoxin and NADPH-adrenodoxin reductase [9]. This constitution of the mitochondrial P450 system looks similar to that of bacterial soluble P450 systems, which consist of soluble P450s, ferredoxin-type iron–sulfur protein, and NADH-ferredoxin reductase [10]. However, mitochondrial P450s in animal cells do not seem to be of prokaryotic origin. Available evidence suggests that a microsome-type P450 was translocated to mitochondria during the course of biological evolution of eukaryotic organisms, and diversified to the present several forms. That event seems to have taken place after animals and plants diverged in the evolution. The presence of P450s in mitochondria is a unique feature of animals; plant mitochondria do not contain P450s.

Although the role of mitochondrial P450s in the metabolism of endogenous steroid substrates and Vitamin D3 has been intensively studied and confirmed, their role in the metabolism of xenobiotic chemicals has been controversial. The presence of P450-catalyzed drug-metabolizing activity in mitochondria was reported at times since the 1980s [11], [12], and the existence of some microsome-type P450s in the mitochondria from various animal tissues including liver and brain has also been reported [13], [14], [15]. It seems that some microsomal P450s are sorted to mitochondria under certain conditions [13], [16]. However, the physiological significance of the contribution of mitochondrial P450s to the metabolism of xenobiotic chemicals in animal tissues is still controversial.