β2-Adrenoceptor agonists in the regulation of mitochondrial biogenesis

The stimulation of mitochondrial biogenesis (MB) via cell surface G-protein coupled receptors is a promising strategy for cell repair and regeneration. Here we report the specificity and chemical rationale of a panel of β₂-adrenoceptor agonists with regards to MB. Using primary cultures of renal cells, a diverse panel of β₂-adrenoceptor agonists elicited three distinct phenotypes: full MB, partial MB, and non-MB. Full MB compounds had efficacy in the low nanomolar range and represent two chemical scaffolds containing three distinct chemical clusters. Interestingly, the MB phenotype did not correlate with reported receptor affinity or chemical similarity. Chemical clusters were then subjected to pharmacophore modeling creating two models with unique and distinct features, consisting of five conserved amongst full MB compounds were identified. The two discrete pharmacophore models were coalesced into a consensus pharmacophore with four unique features elucidating the spatial and chemical characteristics required to stimulate MB.     

Degradation of rat hepatic cytochrome P-450 heme by 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine to irreversibly bound protein adducts

Administration of 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP) (a structural analog of the dihydropyridine Ca2+ antagonists) to untreated, phenobarbital-, or dexamethasone-pretreated rats results in time-dependent losses of hepatic cytochrome P-450 content. Functional markers for various cytochrome P-450 isozymes have permitted the identification of P-450h, P-450 PB-1/k, and P-450p as the isozymes inactivated preferentially by the drug. DDEP-mediated cytochrome P-450 destruction may be reproduced in vitro, is most prominent after pretreatment of rats with dexamethasone, pregnenolone 16 alpha-carbonitrile or phenobarbital, and is blocked by triacetyloleandomycin. These findings together with the observation that DDEP markedly inactivates hepatic 2 beta- and 6 beta-testosterone hydroxylase and erythromycin N-demethylase tend to indict the steroid-inducible P-450p isozyme as a key protagonist in this event. The precise mechanism of such DDEP-mediated P-450p heme destruction is unclear, but involves prosthetic heme alkylation of the apocytochrome at its active site in what appears to be a novel mechanism-based "suicide" inactivation. Such inactivation appears to involve fragmentation of the heme to reactive metabolites that irreversibly bind to the protein, but the chemical structure of the heme-protein adducts is yet to be established. Intriguingly, such DDEP-mediated P-450p destruction in vivo also results in accelerated loss of immunochemically detectable apocytochrome P-450p. It remains to be determined whether or not this loss is due to enhanced proteolysis triggered by the structural modification of the apocytochrome.     

Isotopically labeled chlorobenzenes as probes for the mechanism of cytochrome P-450 catalyzed aromatic hydroxylation

Noncompetitive and competitive intermolecular deuterium isotope effects were measured for the cytochrome P-450 catalyzed hydroxylation of a series of selectively deuterated chlorobenzenes. An isotope effect of 1.27 accompanied the meta hydroxylation of chlorobenzene-2H5 as determined by two totally independent methods (EC-LC and GC-MS assays). All isotope effects associated with the meta hydroxylation of chlorobenzenes-3,5-2H2 and -2,4,6-2H3 were approximately 1.1. In contrast, competitive isotope studies on the ortho and para hydroxylation of chlorobenzenes-4-2H1, -3,5-2H2, and -2,4,6-2H3 resulted in significant inverse isotope effects (approximately 0.95) when deuterium was substituted at the site of oxidation whereas no isotope effect was observed for the oxidation of protio sites. These results eliminate initial epoxide formation and initial electron abstraction (charge transfer) as viable mechanisms for the cytochrome P-450 catalyzed hydroxylation of chlorobenzene. The results, however, can be explained by a mechanism in which an active triplet-like oxygen atom adds to the pi system in a manner analogous to that for olefin oxidation. The resulting tetrahedral intermediate can then rearrange to phenol directly or via epoxide or ketone intermediates.     

An electron microscopic study of Babesia microti invading erythrocytes

Summary Intracellular sporozoan parasites invade the host cell through the invagination of the plasma membrane of the host and a vacuole is formed which accommodates the entering parasite. The vacuole may disappear and the invaginated membrane of the host then becomes closely apposed to that of the parasite's own membrane. As a result the parasite is covered by two membranes. Members of the class Piroplasmea differ from other Sporozoa in that their trophozoites are covered by a single membrane. By screening numerous sections of intraerythrocytic Babesia microti belonging to the class Piroplasmea, it was found that merozoites of Babesia enter the erythrocytes of hamsters in the same way as those of other Sporozoa. When a merozoite touches the red blood cell with its anterior end it becomes attached to the membrane of the host, which starts to invaginate and a parasitophorous vacuole is formed. The vacuolar space disappears rapidly and the membrane of the vacuole and that of the parasite become closely adjacent. At this stage the parasite is surrounded by two plasma membranes. The outer membrane derived from the invaginated host membrane disintegrates quickly and the parasite is left with a single membrane throughout its life span.Supported by Grant AI 08989 from the U.S. Public Health Service. The excellent technical assistance of Ms. Renata Klatt is gratefully acknowledged     

Improved non-parametric statistical methods for the estimation of Michaelis-Menten kinetic parameters by the direct linear plot.

The theoretical basis for the direct linear plot [Eisenthal & Cornish-Bowden (1974) Biochem. J. 139, 715-720], a non-parametric statistical method for the analysis of data-fitting the Michaelis-Menten equation, was reinvestigated in order to accommodate additional experimental designs and to provide estimates of precision more directly comparable with those obtained by parametric statistical methods. Methods are given for calculating upper and lower confidence limits for the estimated parameters, for accommodating replicate measurements and for comparing the results of two separate experiments. Factors that influence the proper design of experiments are discussed.     

Chemical synthesis, absolute configuration, and stereochemistry of formation of 10-hydroxywarfarin: a major oxidative metabolite of (+)-(R)-warfarin from hepatic microsomal preparations

The synthesis of a diastereomerically pure 10-hydroxywarfarin [4-hydroxy-3-(2-hydroxy-3-oxo-1-phenylbutyl)-2H-1 benzopyran-2-one] was accomplished in three steps from racemic warfarin. The relative configuration of the synthetic product was established by conversion to a cyclic derivative followed by NMR and X-ray diffraction analysis. Absolute stereochemistry was determined by enzymatic conversion of either of the pure enantiomers of warfarin to a 10-hydroxy metabolite of known relative configuration. Metabolic formation of 10-hydroxywarfarin was studied using hepatic microsomal preparations from female rats and man. The formation of 10-hydroxywarfarin catalyzed by hepatic microsomes from both dexamethasone-treated rats and man was highly stereoselective [(R)/(S): 3.4-9.0] for (R)-warfarin. In contrast, little stereoselectivity was observed in reactions catalyzed by untreated rat liver microsomes. The resultant stereochemistry at the site of oxidation was also found to be highly dependent on substrate stereochemistry. (R)-Warfarin gave (9R;10S)-10-hydroxywarfarin with only a trace of the (9R;10R) isomer irrespective of which enzyme preparation was used for catalysis, while (S)-warfarin gave (9S;10R)-10-hydroxywarfarin with only a trace of the (9S;10S) isomer, again irrespective of which enzyme preparation was used for catalysis.