Previous results demonstrating homotropic activation of human UDP-glucuronosyltransferase (UGT) 1A1-catalyzed estradiol-3-glucuronidation led us to investigate the effects of 16 compounds on estradiol glucuronidation by human liver microsomes (HLM). In confirmation of previous work using alamethicin-treated HLM pooled from four livers, UGT1A1-catalyzed estradiol-3-glucuronidation demonstrated h...

Argininosuccinase from normal human liver was purified and characterized. The properties of this enzyme were compared to those of argininosuccinase from bovine liver. The molecular weight of the native human liver enzyme is 200,000 as determined by gel filtration. Polyacrylamide disc gel electrophoresis of the enzyme, dissociated by sodium dodecyl sulfate, indicated that it exists as a tetramer...

The membrane adenosine triphosphatase from SfrepfoCOCCUS faecalis has been purified by heat treatment, gel filtration through Agarose, and repeated chromatography on diethylaminoethyl cellulose. The purified ATPase appears to be homogeneous as judged from gel electrophoresis and sedimentation velocity studies. Sedimentation equilibrium studies, which are presented in the succeeding paper, also ...

The kinetic properties of purified yeast pyruvate kinase were investigated. The enzyme showed cooperative kinetics toward the essential activating monovalent cations K+ and NH4+, Mg2+, and phosphoenolpyruvate. Fructose 1,6diphosphate, which yielded homotropic cooperative kinetics and did not affect maximal velocity, transformed the sigmoidal kinetics of Kf and NH4+, Mgz+, and phosphoenolpyruvat...

Several mammalian cytochrome P450 (P450) isoforms demonstrate homotropic cooperativity with a number of substrates, including steroids and polycyclic aromatic hydrocarbons. To identify structural factors contributing to steroid and polycyclic aromatic hydrocarbon binding to P450 enzymes and to determine the location of the allosteric site, we investigated interactions of the macrolide hydroxyla...

BACKGROUND Bell-shaped and terraced dose-response relations have been observed in single ligand application for enzymes, carriers, transporters, G protein-coupled receptors as well as for other receptive units. It seems that there is still a need for new models as analytical tools for such dose-responses, especially in the light of expanding di- and multi-merization of the receptive units for f...

As determined by equilibrium dialysis, bovine liver argininosuccinase of molecular weight 202,000 binds 4 mol of argininosuccinate or arginine/mol of enzyme. Negative homotropic interactions occur in the binding of both ligands at 0.15 ionic strength in the presence of phosphate. Argininosuccinate binds to two sites (Kdiss 1.6 times 10(-5) M) and four sites (Kdiss 2.9 times 10(-4) M) at low and...

Functional positive cooperative activation of the extracellular calcium ([Ca(2+)]o)-sensing receptor (CaSR), a member of the family C G protein-coupled receptors, by [Ca(2+)]o or amino acids elicits intracellular Ca(2+) ([Ca(2+)]i) oscillations. Here, we report the central role of predicted Ca(2+)-binding site 1 within the hinge region of the extracellular domain (ECD) of CaSR and its interacti...

Interactions between the UGT2B7-catalyzed glucuronidation of zidovudine (AZT), 4-methylumbelliferone (4MU), and 1-naphthol (1NP) were analyzed using multisite and empirical kinetic models to explore the existence of multiple substrate and effector binding sites within this important drug metabolizing enzyme. 4MU and 1NP glucuronidation by UGT2B7 exhibit sigmoidal kinetics characteristic of homo...