A cytochrome P450 expressed in pig liver was cloned by polymerase chain reaction using oligonucleotide primers based on amino acid sequences of the purified taurochenodeoxycholic acid 6␣-hydroxylase. This enzyme catalyzes a 6␣-hydroxylation of chenodeoxycholic acid, and the product hyocholic acid is considered to be a primary bile acid specific for the pig. The cDNA encodes a protein of 504 amino acids. The primary structure of the porcine taurochenodeoxycholic acid 6␣-hydroxylase, designated CYP4A21, shows about 75% identity with known members of the CYP4A subfamily in rabbit and man. Transfection of the cDNA for CYP4A21 into COS cells resulted in the synthesis of an enzyme that was recognized by antibodies raised against the purified pig liver enzyme and catalyzed 6␣-hydroxylation of taurochenodeoxycholic acid. The hitherto known CYP4A enzymes catalyze hydroxylation of fatty acids and prostaglandins and have frequently been referred to as fatty acid hydroxylases. A change in substrate specificity from fatty acids or prostaglandins to a steroid nucleus among CYP4A enzymes is notable. The results of mutagenesis experiments indicate that three amino acid substitutions in a region around position 315 which is highly conserved in all previously known CYP4A and CYP4B enzymes could be involved in the altered catalytic activity of CYP4A21.

The microsomal triglyceride transfer protein (MTTP) is essential for formation of apolipoprotein B (apoB)-containing lipoproteins. The liver and the intestine have the highest tissue expression of MTTP and secrete triacylglycerol (TAG)-rich VLDLs and chylomicrons, respectively ( 1 ). MTTP is also expressed in other cells such as cardiomyocytes ( 2 ) and macrophages ( 3 ). The heart secretes apoB100-containing lipoproteins and it has been proposed that cardiac lipoprotein secretion protects the heart against accumulation of lipids that are toxic to the myocardium ( 4 ). This theory is supported by the fi nding that MTTP exAbstract Promoter polymorphisms in microsomal triglyceride transfer protein ( MTTP ) have been associated with decreased plasma lipids but an increased risk for ischemic heart disease (IHD), indicating that MTTP infl uences the susceptibility for IHD independent of plasma lipids. The objective of this study was to characterize the functional promoter polymorphism in MTTP predisposing to IHD and its underlying mechanism. Use of pyrosequencing technology revealed that presence of the minor alleles of the promoter polymorphisms -493G>T and -164T>C result in lower transcription of MTTP in vivo in the heart, liver, and macrophages. In vitro experiments indicated that the minor -164C allele mediates the lower gene expression and that C/ EBP binds to the polymorphic region in an allele-specifi c manner. Furthermore, homozygous carriers of the -164C were found to have increased risk for IHD as shown in a case-control study including a total of 544 IHD patients and 544 healthy control subjects. We concluded that carriers of the minor -164C allele have lower expression of MTTP in the heart, mediated at least partly by the transcription factor CCAAT/enhancer binding protein, and that reduced concentration of MTTP in the myocardium may contribute to IHD upon ischemic damage. -Aminoff, A

Background: Peroxisome proliferator-activated receptor delta (PPARδ) is a member of the nuclear receptor superfamily. Numerous studies have aimed at unravelling the physiological role of PPARδ as a transcriptional regulator whereas the regulation of PPARδ gene expression has been less studied.

The present review aims to give an overview of the cytochrome P450 8B (CYP8B) and cytochrome P450 4A (CYP4A) subfamilies in relation to biosynthesis of bile acids, in particular trihydroxy bile acids. Trihydroxy bile acids are basically required in most species and have an impact on cholesterol and lipid metabolism. The primary trihydroxy bile acid in most mammals is cholic acid. Some species produce other important trihydroxy bile acids, for example the adult pig which produce hyocholic acid instead of cholic acid. The position of the third hydroxyl group in cholic acid and hyocholic acid, 12alpha or 6alpha position, respectively, has a profound effect on the hydrophilic-hydrophobic property of the trihydroxy bile acids. The CYP8B subfamily is required for introduction of the 12alpha-hydroxyl group in cholic acid biosynthesis. The enzyme responsible for 6alpha-hydroxylation in hyocholic acid biosynthesis, however, varies among species. This review will discuss, in particular, porcine members of the CYP8B and CYP4A subfamilies because interesting findings regarding members of these subfamilies have recently been recognized in this species. CYP8B1 was for a long time believed to be absent in the pig but was recently found to be expressed in fetal pig liver. The enzyme catalyzing the 6alpha-hydroxylation in hyocholic acid biosynthesis in pig was found to be an atypical member of the CYP4A subfamily, denoted CYP4A21. The review presents bile acid biosynthesis in view of these findings and discusses physiochemical properties and developmental-dependent aspects related cholic acid and hyocholic acid biosynthesis.

No abstract