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The formation of sulpho-conjugates of 3,4-dihydroxyphenylethylamine (dopamine) and related compounds was examined in preparations of rat tissues. Liver high-speed-supernatant preparations readily transferred sulphate from adenosine 3'-phosphate 5'-sulphato-phosphate to dopamine under standard conditions. The main product was identified as the 3-O-sulphate. The preparation also sulphated the 3- and 4-methoxy derivatives but to a lesser extent (44% and 95% respectively) relative to dopamine. Brain preparations possessed only half the activity of liver but formed both the 3- and 4-O-sulphates in the molar ratio of 1.7:1. l-3,4-Dihydroxyphenylalanine (l-dopa) in both tissue preparations did not yield any significant amount of sulpho-conjugate when the dopa decarboxylase present was inhibited. The sulphotransferase activity of preparations was doubled in the presence of dithiothreitol and it was concluded that l-tyrosine methyl ester sulphotransferase was the enzyme involved. A method for the preparation of authentic dopamine 3-O-sulphate and 4-O-sulphate was developed.

BIOCHEMICAL SOCIETY TRANSACTIONS involved incubations for a standard period of 32 h. Data in Fig. l ( b ) indicate that the production rate of IGF-1 was sensitive to the amount of glucose and amino acids in the medium. Altering the concentration of glucose plus amino acids, or glucose alone (10 to 130% and 25 to 200%, respectively of the standard concentration) resulted in exponential changes in IGF-1 production. When amino acid concentration was varied independently of glucose, between the range 15 to 100% similar responses were observed, but above 100% production was reduced, probably because of pH changes in the medium.Although previous reports indicate that GH increases IGF-1 production [3] and IGF-1 mRNA concentrations [4] in rat hepatocytes as well as plasma IGF-1 concentrations in growing sheep [S], and pigs [ 6 ] , the present experiment showed no difference in IGF-1 production in the presence of 0, 5 and 200 pg/l rbGH. This finding is in agreement with earlier observations of Postel-Vinay and colleagues [7].Based on these observations it would appear that IGF-I production by rat liver cells is directly influenced by nutrient availability but is not responsive to GH.Several groups of workers have investigated the relationship between drug metabolism in isolated hepatocytes and that in vivo [l-31. There is a growing interest in using isolated hepatocytes to predict the intrinsic metabolic clearance of drugs in vivo [4]. Conventional kinetic studies with hepatocytes can reveal the apparent maximum rate of metabolism ( V ) and the substrate (drug) concentration required to give half the apparent maximum rate (K,,,). Under first-order conditions V and K , can be related to intrinsic clearance (CIln,,) of substrate by the relationship Clint. = V/K,.Intrinsic clearance may also be determined in vivo by pharmacokinetic analysis of circulating drug concentration.The relationship CIP,a,ma = CIKena! + CIHepallc allows the calculation of hepatic clearance which, together with the free fraction of drug in plasma, can provide data on intrinsic clearance in vivo. In this study we compare the intrinsic clearance calculated from apparent V and K , values in isolated hepatocytes with that from pharmacokinetic analysis for the H,-receptor antagonist loxtidine hemisuccinate, a compound with a low hepatic extraction ratio.In laboratory animals 151 and man loxtidine undergoes metabolism by several routes and shows significant species differences. In rat the major route of metabolism is Ndealkylation, but in dog loxtidine is extensively conjugated with glucuronic acid. In man loxtidine is excreted predominantly unchanged although a minor phase I metabolite was observed. We have investigated the metabolism of loxtidine in hepatocytes isolated from rat, dog and man and have observed the same pattern of metabolites as in vivo for each species. Conventional kinetic analysis was used to calculate apparent K,, and V which were then used to calculate intrinsic clearance. Intrinsic clearance was also calculated from pharmacokinetic ...

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