[Severe deglutition disorders in a patient with a mediastinal space-occupying lesion].
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Adult↗
Biomedical subjects
G M Robertz
Publications and source records attributed to G M Robertz.
Evidence for altered catalytic properties of the cytochrome P-450 involved in sparteine oxidation in poor metabolizers.
Sparteine metabolism was studied in human liver microsomes from nine extensive ([EM] urinary metabolic ratio [MR] less than 20) and four poor metabolizers ([PM] MR greater than 20). The formation of 2- and 5-dehydrospartein displayed monophasic Michaelis-Menten kinetics. In livers from PMs the formation of the major sparteine metabolite 2-dehydrosparteine was characterized by more than a thirtyfold increase in Michaelis-Menten constant (Km)(1880 +/- 1044 mumol/L) as compared with EM subjects with an MR less than 1 (58.3 +/- 38.8 mumol/L). EM subjects with an MR greater than 3 who may constitute heterozygous metabolizers showed Km values in between (658 +/- 301 mumol/L). There was no difference in maximum rate of metabolism (Vmax) of 2-dehydrosparteine formation between EM and PM subjects (101 +/- 39 vs. 86 +/- 52 pmol/min/mg). The formation of 5-dehydrosparteine exhibited a Km of 100 +/- 123 mumol/L similar to the Km of 2-dehydrosparteine formation. The urinary MR correlated positively with the Km for 2-dehydrosparteine formation. The intrinsic clearance for 2-dehydrosparteine showed a highly significant negative correlation with the MR. The pronounced differences in Km together with the significant correlation between Km and MR with no marked differences in Vmax between phenotypes suggest that the impaired oxidation capacity in PM subjects is more likely the result of a P-450 isozyme with altered catalytic properties rather than a decreased amount of enzyme.
Adult↗
Characterization of a common genetic defect of cytochrome P-450 function (debrisoquine-sparteine type polymorphism)--increased Michaelis is Constant (Km) and loss of stereoselectivity of bufuralol 1'-hydroxylation in poor metabolizers.
In order to define the mechanism of the debrisoquine-sparteine type genetic polymorphism of drug oxidation we studied the kinetics of bufuralol 1'-hydroxylation in liver microsomes from extensive and poor metabolizers and in a purified reconstituted human cytochrome P-450 isozyme with high activity for bufuralol 1'-hydroxylation, P-450[buf]. In extensive metabolizer microsomes the enzymatic reaction displayed apparent Michaelis-Menten kinetics and the (+)-isomer was preferentially metabolized. By contrast, the enzymatic reaction in poor metabolizer microsomes was characterized by a 4- to 5-fold increase in Km and by a loss of stereoselectivity. In a non-membraneous reconstituted system containing NADPH cytochrome P-450 reductase, a NADPH regenerating system and phospholipids, P-450[buf] exhibited an almost complete substrate stereoselectivity for (+)-isomer 1'-hydroxylation. It is concluded that the purified cytochrome P-450[buf] is the target of the debrisoquine-sparteine type oxidation polymorphism and that poor metabolizers have a quantitative or qualitative deficiency of this isozyme.
Cytochrome P-450 Enzyme System↗
Endogenous ligand(s) decrease drug--protein binding in uremic sera: a fluorescence probe study.
Human serum albumin (HSA) was isolated and purified (greater than 97% purity) from normal sera, from sera of patients with severe chronic renal insufficiency and from sera to which a strongly protein bound acidic drug--clofibrinic acid--was added as a model ligand. The binding properties were evaluated using dansylglycine as a fluorescent probe. Data were analyzed according to Scatchard, the binding constants were calculated by least square approximation. The binding of dansylglycine to HSA from uremic sera was substantially decreased, reflected mainly by a lower product n1 . K1, as was the binding of dansylglycine to HSA from model sera containing clofibrinic acid. The binding was restored to almost normal when HSA was treated with charcoal. It is concluded that the impaired binding of many mostly acidic drugs to HSA in uremia is due to the presence of endogenous ligands. In addition a minor contribution by changes in HSA structure cannot be excluded.
Adult↗
Substrate specificity of the form of cytochrome P-450 catalyzing the 4-hydroxylation of debrisoquine in man.
In the present study we have investigated the substrate specificity of the form of cytochrome P-450 catalyzing the 4-hydroxylation of debrisoquine in man by analyzing the kinetics of inhibition of this activity by potential alternative substrates for the enzyme. All three compounds for which there is good in vivo evidence for an association between their metabolism and the debrisoquine oxidation polymorphism (viz., sparteine, guanoxan and phenformin) were potent competitive inhibitors of the reaction. The Ki for sparteine was 85 microM, for guanoxan it was 30 microM, and for phenformin it was 205 microM. Two compounds, acetanilide and antipyrine, for which the in vivo evidence was against an association between their metabolism and that of debrisoquine, were weak, noncompetitive inhibitors of debrisoquine 4-hydroxylase activity. The Ki values were 1.23 mM and 19.3 mM, respectively. Two additional compounds, tolbutamide and amylobarbitone, for which the in vivo evidence was also against an association between their metabolism and the debrisoquine oxidation polymorphism, did not appreciably inhibit the reaction. In fact, amylobarbitone caused a slight stimulation of activity. It is concluded that debrisoquine 4-hydroxylase is a specific form of cytochrome P-450 with a well-defined substrate specificity. Furthermore, it should be possible to identify compounds that might be subject to an oxidation polymorphism prior to the exposure of any subjects to the compound.
Antipyrine↗