EVIDENCE FOR METABOLISM OF SCAVENGED STEROLS BY THE P CARINII SAM:SMT: TRANSMETHYLATION OF DESMOSTEROL
Pneumocystis, an opportunistic fungal protist, causes a type of pneumonia in immunocompromised individuals such as AIDS patients. Rat-derived P. carinii and human-derived P. jiroveci contain a large number of sterols with C-24 alkyl groups. S-Adenosyl-L-methionine:sterol C-24 methyl transferase (SAM:SMT) is the enzyme that transfers methyl groups from SAM to the C-24 position of the sterol side chain. An alkyl group at the C-24 sterol side chain position appears to be essential for the organism’s survival and proliferation. Thus SAM:SMT, which is absent in mammals, is an attractive target for chemotherapeutic attack against the pathogen. The P. carinii erg6 gene that codes for SAM:SMT has been sequenced, cloned, and the protein expressed in E. coli. Since bacteria do not synthesize sterols, and do not have SAM:SMT, the P. carinii erg6 gene product expressed in E. coli would only transmethylate exogenously provided sterol substrates. The P. carinii recombinant SAM:SMT prefers lanosterol, a central intermediate in sterol biosynthesis, over zymosterol. Some SAM:SMT, e.g. Sachharomyces cerevisiae and Candida albicans, do not bind lanosterol and prefer other sterol substrates produced from lanosterol. In the present study, P. carinii SAM:SMT was shown to also catalyze the methylation of desmosterol. Desmosterol is the direct precursor of cholesterol and these are available in the rat lung for scavenging by P. carinii. Methylation of the desmosterol side chain produced two 24-alkylsterol products; 24-methylenecholesterol and 24-ethylidenecholesterol. The ability of the recombinant P. carinii SAM:SMT to productively bind desmosterol and produce two methylation products suggest that the enzyme has broad substrate specificity and is consistent with the suggestion that it is a unique fungal SAM:SMT enzyme.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:p carinii sam smt sterols methylation
Date of Publication:01/01/2004