No abstract

Thiol methyltransferase (EC 2.1.1.9, TMT) activity was measured with 2-mercaptoethanol in the microsomal fraction of 12 placenta and 31 fetal and 33 adult liver specimens. TMT activity (nmol/min incubation/mg protein; mean ± SD) was 0.61 ± 0.25 (placenta), 0.74 ± 0.45 (fetal liver), and 4.51 ± 2.29 (adult liver). TMT activity was also measured in extrahepatic tissues and it was about one order of magnitude lower in fetal lungs, kidney and intestine as compared with the fetal liver. A similar distribution pattern was also observed in adult tissues except that in the kidney TMT activity was one third of the hepatic one. Studies of enzyme kinetics showed that fetal and adult hepatic TMT obeyed non-Michaelis-Menten kinetics when 2-mercaptoethanol was the varying substrate. Average values of Km for the higher and lower affinity phases were 0.03 and 14.05 mmol/l, respectively (fetal liver) and 0.005 and 14.57 mmol/l, respectively (adult liver). This paper shows that TMT develops prenatally and its distribution pattern is consistent with that of other microsomal enzymes, being preferentially associated with the liver both in the human fetus and in adult subject.

The range of hepatic rate of MPA glucuronidation is narrow relative to those of ethinyloestradiol, testosterone and zidovudine glucuronidation obtained from literature. The Ki value of NA is one order of magnitude lower than the K(m) for MPA in non-inhibited samples. This indicates that the inhibitor affinity for glucuronosyl transferase is greater than that of the substrate. The range of variation of NA IC(50) values is narrow.

The protein binding of furosemide was studied in the serum from 8 umbilical cords, in 51 children (aged between 2 weeks and 13.5 years) and in the plasma of 10 volunteers (aged between 28 and 42 years). The drug was added to the buffer to give a final concentration of 2 pg/ml. The unbound fraction of furosemide was 2.5 ± 0.1 % (cord serum) and 1.7 ± 0.7% (adult plasma). These figures are different at a level of 0.001. The unbound fraction of furosemide reached the adult values during the 1st year of life. A correlation (level of significance >0.01) was found between the unbound fraction and the age during the first 6 months of life. The furosemide binding kinetics were studied in 3 cord serum and in 3 adult plasma samples. The concentration of the drug in the buffer ranged between 1 and 16 pg/ml. The kinetic constants (mean ± SEM) were: association constant (K = 10^5 M^-) 2.4 ± 0.3 (cord serum), 2.0 ± 0.2 (adult plasma); the number of binding sites per gram protein (n = 10^-6) was 3.2 ± 0.5 (cord serum) and 3.9 ± 0.7 (adult plasma). When the concentration of furosemide was increased up to 200 pg/ml buffer, the free fraction of the drug increased up to 4.8 ± 0.2% (cord serum) and 2.9 ± 0.4% (adult plasma).

The protein binding of furosemide was studied in the serum from 7 umbilical cords and 7 healthy adult subjects in presence or absence of bilirubin. In cord serum, the unbound fraction of furosemide (mean ± SD) was 2.32 ± 0.14 (control), 2.94 ± 0.26 (200 μM bilirubin) (p < 0.001) and 3.52 ± 0.38 (400 μM bilirubin) (p < 0.001). Percent increase (mean ± SD) of the unbound furosemide was 22 ±11 and 51 ± 12 after 200 and 400 μM bilirubin, respectively. In adult serum, the unbound fraction of furosemide was 1.69 ± 0.21 (control), 1.93 ± 0.24 (200 μM bilirubin) and 2.15 ± 0.38 (400 μM bilirubin). Percent increase of the unbound fraction was 14 ± 8 and 28 ± 16 after 200 and 400 μM bilirubin, respectively. Binding of furosemide was also studied in 5 cord and adult serum specimens previously dialysed for 18 h at 4 °C. Unbound furosemide in dialyzed serum was 1.39 ± 0.05 (cord) and 1.25 ± 0.04 (adult). Cord serum contains dialyzable compounds that enhance the unbound fraction of furosemide. Displacement effect of bilirubin was unchanged by dialysis and it was significantly higher in cord than adult serum. The different displacing effect of bilirubin might suggest the presence of qualitatively different albumin in cord serum.