ROSFIT: an enzyme kinetics nonlinear regression curve fitting package for a microcomputer

Partial characterization of the microsomal and solubilized hypothalamic progesterone 5 alpha-reductase

Microsomal progesterone 5 alpha-reductase activity from female rat hypothalamus has been solubilized and partially characterized in terms of kinetic and physical properties. The solubilization of progesterone 5 alpha-reductase has been accomplished through the use of a digitonin/KCL-extraction. Both the microsomal and solubilized enzyme activities exhibit similar kinetic and physical characteristics. These include their apparent Km for progesterone (Microsomal Km = 113 +/- 11 nM; solubilized Km = 144 +/- 20 nM) and their affinity (approximately 7 nM) for the 5 alpha-dihydroprogesterone analog, 4-aza-4-methyl-5 alpha-pregnane-3,20-dione, which is a potent inhibitor of progesterone 5 alpha-reduction. Both activities are inhibited by divalent cations (Zn2+ and Cu2+) and the sulfhydryl-blocking agent p-chloromercuribenzoic acid. Studies aimed at optimizing isolation and assay conditions for the hypothalamic progesterone 5 alpha-reductase indicate that the microsomal activity is enhanced in the presence of monovalent cations (particularly K+ and Li+) and the metal chelator EDTA, but is unaffected by the sulfhydryl reducing agent dithiothreitol. The activity of the solubilized enzyme is also enhanced by EDTA but slightly stimulated by dithiothreitol. Analysis of hypothalamic progesterone 5 alpha-reduction for possible flavin involvement (as a hydride carrier between NADPH and the steroid) indicates that the enzyme activity is decreased by high levels of flavins, flavin analogs and riboflavin deficiency.

RECFIT: a microcomputer-based nonlinear regression curve-fitting package for the quantitative resolution of beta-adrenoceptor subtypes and estimation of nonspecific binding

A dedicated nonlinear regression based curve-fitting packages has been developed for quantitative analysis of beta-adrenoceptor subtypes. A feature of this package is the provision to obtain initial parameter estimates using a conversational graphical technique where the user is prompted for parameter estimates, and the resulting curve is displayed over the data. Data are then fitted to a one- or two-binding site model with user-selected weighting and constraints on the parameters. A novel feature is the provision to estimate the nonspecific binding component in the assays as a parameter in the model. The printout for each model consists of the parameters and their standard deviations, estimates of the goodness of fit, an analysis of residuals, and a graph of the data points overlayed with the fitted curve. The nonlinear regression algorithm is based on that of Gauss-Newton. When unweighted, the values determined by RECFIT are essentially the same as those found using the BMDPAR programs on an ICL 2976 mainframe computer. The implementation is reasonably efficient; a typical run for a two-site fit, using nine data points and estimating nonspecific binding, took a total time of about 8 min, excluding data entry and derivation of initial estimates, 4 min for the fitting, 30 sec for graph generation, and 2-3 min for printing of the graph and data.

Galactose elimination kinetics in perfused rat liver after two thirds hepatectomy

Galactose elimination kinetics was studied in isolated perfused livers (single pass) from rats subjected to two thirds partial hepatectomy 6 h earlier or immediately before investigation. No difference was found between the two groups in the maximum galactose elimination rate or in the half saturation galactose concentration. This is in contrast with in vivo observation of restored galactose elimination capacity 6 h after two thirds hepatectomy. The discrepancy indicates that extrahepatic factors may be of importance for the metabolic regulation in the liver during regeneration.

S-, N-, and O-glycosyl derivatives of 2-acetamido-2-deoxy-D-glucose with hydrophobic aglycons as potential chemotherapeutic agents and N-acetyl-beta-D-glucosaminidase inhibitors

S-, N-, and O-Glycosyl derivatives of 2-acetamido-2-deoxy-D-glucose with hydrophobic aglycons have been obtained as potential, plasma-membrane active agents. 2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-1-thio-beta-D-glucopyranose (6) was converted into benzyl, diphenylmethyl, triphenylmethyl, and other thioglycosides. Acylation of 6 gave adamantoyl and haloacetyl derivatives. A similar series of N- and O-glycosyl derivatives was obtained from the corresponding NH2-1 and OH-1 analogs of 6, such as O- and N-dinitrophenyl, O- and N-adamantoyl, and N-4-methylbenzylidene derivatives. Several N- and S-glycosyl derivatives were found to inhibit mouse mammary adenocarcinoma (TA3) cells in vitro as well as N-acetyl-beta-D-glucosaminidase from beef liver.

A free derivate program for non-linear regression analysis of enzyme kinetics to be used on small computers

A non-linear regression program, written in BASIC, is described. The program uses the Marquardt 's algorithm as modified by Reich et al. (Eur. J. Biochem., 26 (1972) pp. 368-379). The user only supplies the expression to fit, since the program uses numerical differentiation. It is possible to fit models of 1 substrate, 2 substrates, 1 substrate and 1 inhibitor, and 2 substrates and 1 inhibitor. Likewise, several weighting patterns, as well as a simple or robust regression, can be selected.

Influence of tamoxifen and its N-desmethyl and 4-hydroxy metabolites on rat liver microsomal enzymes

Tamoxifen (Nolvadex; TAM) and its major metabolites, N-desmethyl- (DMT) and 4-hydroxy-tamoxifen (HT), were shown to be potent inhibitors of hepatic cytochrome P-450-dependent mixed function oxidations. From in vitro experiments, all three were found to be potent inhibitors of oxidation of Type-I substrates (ethylmorphine and aminopyrine) and less potent, non-competitive inhibitors of Type-II substrates (aniline and dimethylnitrosamine). TAM, DMT and HT were of essentially equal potency and had a much more pronounced effect on Type-I substrates than on Type-II compounds studied. Their action appears to parallel SKF-525A in type and potency of inhibition produced. Spectral binding studies suggest that TAM and its metabolites exert their effects by occupying the Type-I binding site of cytochrome P-450 and thus limiting the accessibility of other substrates to the active site of the enzyme. TAM (and its metabolites) also inhibits its own metabolism, altering the distribution and elimination half-lives of tamoxifen-derived species. In addition, tamoxifen metabolism was found to be sensitive to the presence of other drugs. These results raise concern regarding the role that continued administration of tamoxifen plays in changing its own disposition as well as in the detoxification of drugs administered with it.

Least squares fit of data to hyperbolic dose-response curves using a programmed minicalculator (TI-59)

Equations of the Michaelis-Menten form are frequently encountered in a number of areas of biochemical and pharmacological research. A program is presented for use on the programmable TI-59 calculator with added printer which performs an iterative least-squares fit of up to 80 data pairs to this equation and estimates the standard deviations and standard errors of the determined parameters. The program assigns equal weights to errors over the entire data range and is thus appropriate for situations in which data precision is independent of amplitude.

A high-performance liquid chromatography method for the assay of cytidine monophosphate-sialic acid synthetase

An HPLC method has been developed for the assay of cytidine monophosphate-sialic acid synthetase (EC 2.7.7.43) using ion-pair chromatography and gradient elution. This procedure permits the assay of alternative substrates and inhibitors of the enzyme and is not subject to the limitation of the colorimetric method. The newly synthesized N-acetyl-9-deoxy-9-fluoro-D-neuraminic acid was found to be a good substrate of the enzyme with a Km of 6.35 mM as compared to 1.84 mM for N-acetylneuraminic acid.