Vitamin P450 activity

Figure 10.16. Overview of P450s involved in key steps of vitamin D activation ...

As with prokaryotic P450s, active site diversity underlies the unique roles of P450s in mamma-han physiology. Structures now are available for several of the enzymes that hydroxylate the aliphatie side chains of cholesterol and vitamin D3. P450 llAl eatalyzes three successive oxy-... 

Fig. 9.29 Overview of P450s involved in key steps of vitamin D activation [47]. (With kind permission from Springer Science + Business Media [149], Fig. 10.16)...

H)2-D3 is a weak agonist and must be modified by hydroxylation at position Cj for full biologic activity. This is accomplished in mitochondria of the renal proximal convoluted tubule by a three-component monooxygenase reaction that requires NADPFl, Mg, molecular oxygen, and at least three enzymes (1) a flavoprotein, renal ferredoxin reductase (2) an iron sulfur protein, renal ferredoxin and (3) cytochrome P450. This system produces l,25(OH)2-D3, which is the most potent namrally occurring metabolite of vitamin D. 

On the way to calcitriol (vitamin D hormone see p.342), another double bond in the B ring of cholesterol is first introduced. Under the influence of UV light on the skin, the B ring is then photochemically cleaved, and the secosteroid cholecalciferol arises (vitamin Dai see p.364). Two Cyt P450-depen-dent hydroxylations in the liver and kidneys produce the active vitamin D hormone (see p. 330). 

Mitochondrial system The function of the mitochondrial cyto chrome P450 monooxygenase system is to participate in the hydroxylation of steroids, a process that makes these hydropho bic compounds more water soluble. For example, in the steroid hormone-producing tissues, such as the placenta, ovaries, testes, and adrenal cortex, it is used to hydroxylate intermediates in the conversion of cholesterol to steroid hormones. The liver uses this system in bile acid synthesis (see p. 222), and the kidney uses it to hydroxylate vitamin 25-hydroxycholecalciferol (vitamin D, see p. 384) to its biologically active 1,25-hydroxylated form. 

Formation of 1,25-diOH D3 Vitamins D2 and D3 are not biologically active, but are converted in vivo to the active form of the D vitamin by two sequential hydroxylation reactions (Figure 28.23). The first hydroxylation occurs at the 25-position, and is catalyzed by a specific hydroxylase in the liver. The product of the reaction, 25-hydroxycholecalciferol (25-OH D3), is the predominant form of vitamin D in the plasma and the major storage form of the vitamin. 25-OH D3 is further hydroxylated at the one position by a specific 25-hydroxycholecalciferol 1 -hydroxylase found primarily in the kidney, resulting in the formation of 1,25-dihydroxycholecalciferol j (1,25-diOH D3). [Note This hydroxylase, as well as the iver 25-hydroxylase, employ cytochrome P450, molecular oxygen, and NADPH.]... 

Vitamin deficiencies, in general, bring about a reduction in monooxygenase activity, although exceptions can be noted. Riboflavin deficiency causes an increase in P450 and... 

Jurutka PW, Thompson PD, Whitfield GK, et al. Molecular and functional comparison of 1,25-dihydroxy vitamin D(3) and the novel vitamin D receptor ligand, lithocholic acid, in activating transcription of cytochrome P450 3A4. J Cell Biochem 2005 94(5) 917-943. 

Metabolites of PCBs also exert biological effects. The Ah-receptor mediated responses, however, are probably caused by the parent compounds only. The effects of hydroxylated PCBs include inhibition of cytochrome P450-dependent enzyme activities and competitive interference with thyroid hormone and vitamin A metabolism.89,90 Methylsulfonyl-PCBs have been shown to inhibit aryl hydrocarbon hydroxylase activity91 and to elicit phenobarbital-type toxicity and may in fact be responsible for the observed effect presumed to be caused by the parent compound.92... 

Attempts to demonstrate 25-hydroxy-vitamin D3-l-hydroxylase activity in vitro with rat kidney homogenates have been unsuccessful, although chick kidney preparations exhibit such activity. A heat-labile and very potent inhibitor of the hydroxylase has now been found in the rat preparation 322 all fractions of the kidney homogenate contained the factor, but the microsomes were the richest source, and they released the inhibitor during incubation. A similar inhibitor is also present in rat intestine and serum and in pig kidney, and it may well play a regulatory role in the synthesis of 1,25-dihydroxy-vitamin D3.323 Direct spectroscopic and inhibitory evidence for the presence of cytochrome P450 in kidney mitochondria and of its... 

Cholecalciferol 25-hydroxylase is not restricted to the liver kidneys, skin, and gut microsomes also have a cytochrome P450 -dependent enzyme that catalyzes the 25-hydroxylation of cholecalciferol and la-hydroxycholecalciferol, hut not ergocalciferol. Although there is some evidence that calcitriol can reduce the activity of calciferol 25-hydroxylase, it is not known whether this is physiologically important the major factor controlling 25-hydroxylation is the rate of uptake of cholecalciferol into the liver. It is the fate of calcidiol in the kidneys that provides the most important regulation of vitamin D metabolism (Wikvall, 2001). 

The active metabolite of vitamin D, calcitriol, is formed in the proximal tubules of the kidneys from calcidiol. There are three cytochrome P450-dependent enzymes in kidneys that catalyze 1-hydroxylation of calcidiol CYP27A and CYP27 in mitochondria and a microsomal la-hydroxylase, which is ferredoxin-dependent. It is likely that the microsomal enzyme is the most important its synthesis is induced by cAMP in response to parathyroid hormone (Section 3.2.8.2) and repressed by calcitriol (Omdahl et al., 2001 Wikvall, 2001). 

Vitamin E deficiency is also associated with impaired mitochondrial oxidative metabolism and impaired activity of microsomal cytochrome P450-dependent mixed-function oxidases, and hence the metabolism of xenobi-ofics. There is no evidence that vitamin E has any specific role in electron transport in mitochondria or microsomes. Again, changes in membrane lipids and oxidative damage presumably account for the observed metabolic abnormalities. 

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