Susceptibility to murine cholesterol gallstone formation is not affected by partial disruption of the HDL receptor SR-BI

High density lipoprotein (HDL) promotes reverse cholesterol transport from peripheral tissues to the liver where its cholesterol is secreted preferentially into bile. The scavenger receptor class B type I (SR-BI) is believed to play a pivotal role in unloading HDL cholesterol and its ester to hepatocytes. Here, using male SR-BI "att" mice with a dysfunctional mutation in the Sr-b1 promoter, we studied whether approximately 50% of normal SR-BI expression influences gallstone susceptibility in these mice fed a lithogenic diet containing 1% cholesterol, 0.5% cholic acid and 15% butterfat. Our results showed that the disruption of SR-BI expression reduced cholesterol secretion by 37% in the chow-fed state and 10% on the lithogenic diet, and while delaying incidence slightly, did not influence cumulative susceptibility to cholesterol gallstones. The lithogenic diet induced marked increases in biliary cholesterol and phospholipid secretion rates but not of bile salts. Basal expression of hepatic SR-BI protein was dissimilar in both wild-type and SR-BI mice, and remained unaltered in response to the lithogenic diet. By two independent dual isotope methods, intestinal cholesterol absorption was unimpaired by attenuation of the SR-BI which also displays low-density expression on small intestinal enterocytes. We conclude that although HDL cholesterol is a principal source of biliary cholesterol in the basal state, uptake of cholesterol from chylomicron remnants appears to be the major contributor to biliary cholesterol hypersecretion during diet-induced cholelithogenesis in the mouse.     

Transcarboxylase 12S crystal structure: hexamer assembly and substrate binding to a multienzyme core

Transcarboxylase from Propionibacterium shermanii is a 1.2 MDa multienzyme complex that couples two carboxylation reactions, transferring CO(2)(-) from methylmalonyl-CoA to pyruvate, yielding propionyl-CoA and oxaloacetate. The 1.9 A resolution crystal structure of the central 12S hexameric core, which catalyzes the first carboxylation reaction, has been solved bound to its substrate methylmalonyl-CoA. Overall, the structure reveals two stacked trimers related by 2-fold symmetry, and a domain duplication in the monomer. In the active site, the labile carboxylate group of methylmalonyl-CoA is stabilized by interaction with the N-termini of two alpha-helices. The 12S domains are structurally similar to the crotonase/isomerase superfamily, although only domain 1 of each 12S monomer binds ligand. The 12S reaction is similar to that of human propionyl-CoA carboxylase, whose beta-subunit has 50% sequence identity with 12S. A homology model of the propionyl-CoA carboxylase beta-subunit, based on this 12S crystal structure, provides new insight into the propionyl-CoA carboxylase mechanism, its oligomeric structure and the molecular basis of mutations responsible for enzyme deficiency in propionic acidemia.     

Raman spectroscopy of uracil DNA glycosylase-DNA complexes: insights into DNA damage recognition and catalysis

Using off-resonance Raman spectroscopy, we have examined each complex along the catalytic pathway of the DNA repair enzyme uracil DNA glycosylase (UDG). The binding of undamaged DNA to UDG results in decreased intensity of the DNA Raman bands, which can be attributed to an increased level of base stacking, with little perturbation in the vibrational modes of the DNA backbone. A specific complex between UDG and duplex DNA containing 2'-beta-fluorodeoxyuridine shows similar increases in the level of DNA base stacking, but also a substrate-directed conformational change in UDG that is not observed with undamaged DNA, consistent with an induced-fit mechanism for damage site recognition. The similar increases in the level of DNA base stacking for the nonspecific and specific complexes suggest a common enzyme-induced distortion in the DNA, potentially DNA bending. The difference spectrum of the extrahelical uracil base in the substrate-analogue complexes reveals only a small electron density reorganization in the uracil ring for the ground state complex, but large 34 cm(-)(1) downshifts in the carbonyl normal modes. Thus, UDG activates the uracil ring in the ground state mainly through H bonds to its C=O groups, without destroying its quasi-aromaticity. This result is at variance with the conclusion from a recent crystal structure, in which the UDG active site significantly distorts the flipped-out pseudouridine analogue such that a change in hybridization at C1 occurs [Parikh, S. S., et al. (2000) Proc. Natl. Acad. Sci. USA 97, 5083]. The Raman vibrational signature of the bound uracil product differs significantly from that of free uracil at neutral pH, and indicates that the uracil is anionic. This is consistent with recent NMR results, which established that the enzyme stabilizes the uracil anion leaving group by 3.4 pK(a) units compared to aqueous solution, contributing significantly to catalysis. These observations are generally not apparent from the high-resolution crystal structures of UDG and its complexes with DNA; thus, Raman spectroscopy can provide unique and valuable insights into the nature of enzyme-DNA interactions.