Hepatic damage in the rat following administration of thyroxine or triiodothyronine, assessed by measurement of plasma glutathione S-transferase YaYa concentrations

Diminished drug metabolism due to hormonal effects of cobalt mesoporphyrin on flavoproteins

Substituted metalloporphyrins, in addition to their use as pharmacological agents, are used to investigate metabolic pathways by inhibiting cytochrome P-450. We have examined the specificity of this approach with cobalt mesoporphyrin (CoMP). In vivo, CoMP (50 mumol/kg, s.c.) decreased rat hepatic microsomal cytochrome P-450, NADPH cytochrome P-450 reductase, benzphetamine N-demethylase (BZPH) activity, and thyroid hormones by > 50%, all of which returned to control levels after 45 days; testosterone levels were also reduced at this dose. The half-life of CoMP was 18 days, which is consistent with this sustained effect. At 10 mumol/kg of CoMP, the reductase activity was decreased, but cytochrome P-450 was unchanged. An effect of residual CoMP on the reductase was ruled out as the CoMP content of tissue fractions was not high enough to inhibit directly the reductase activity (even after 50 mumol CoMP/kg). However, immunoblots indicated a lower level of immunoreactive reductase protein following treatment. After 8 weekly doses of 1 mumol CoMP/kg, BZPH activity was 39% less than control but neither P-450 content nor reductase activity was significantly changed. The P-450 content and reductase activity in rabbits were much less affected by CoMP, perhaps due to differences in the disposition of CoMP. Thyroidectomy decreased reductase activity in rats to an extent that was seen with CoMP at 50 mumol/kg; CoMP treatment of thyroidectomised rats did not further decrease reductase activity. Supplementation with thyroid hormone blocked the CoMP-related decrease. The flavin-containing monooxygenase was decreased by CoMP and by castration, and the decrease was not blocked by the thyroid hormone supplement. Thus in the rat, the CoMP-related decreases in thyroid hormone and testosterone decrease flavoproteins that support or mediate monooxygenase activities. This is contrary to the reported specificity of this class of compound as inhibitors of cytochrome P-450.

Restoration of normal thyrotrophin secretion reduces the abnormally high serum glutathione S-transferase levels found in patients receiving thyroxine replacement therapy

The peripheral tissue thyroid status of 12 patients receiving thyroxine replacement therapy was investigated both when pituitary secretion of TSH was suppressed and later, when on a lower dose of thyroxine that restored thyrotroph responsiveness. Heart rate and various analytes in serum known to be sensitive to thyroid status were measured in addition to TSH by immunoradiometric assay. Initially, the serum T4 concentration was raised in seven patients and free T4 raised in nine; all patients had normal T3 concentrations. Later, on the lower dose of thyroxine, most patients had concentrations of thyroid hormones within reference limits. Concentrations of the liver-specific form of glutathione S-transferase (GST) in serum decreased (P less than 0.01) after the reduction in thyroxine dose; abnormally high GST levels, found in eight patients when TSH was suppressed, returned to normal in six of these patients when normal basal and TRH-stimulated TSH concentrations had been restored. The response of the pituitary to excess thyroxine may be more representative of other tissues (e.g. the liver) than previously thought.

The effects of selenium and copper deficiencies on glutathione S-transferase and glutathione peroxidase in rat liver

Selenium (Se) deficiency in rats produced significant increases in the activity of hepatic glutathione S-transferase (GST) with 1-chloro-2,4-dinitrobenzene as substrate and in various GST isoenzymes when determined by radioimmunoassay. These changes is GST activity and concentration were associated with Se deficiency that was severe enough to provoke decreases of over 98% in hepatic Se-containing glutathione peroxidase activity (Se-GSHpx). However, decreases in hepatic Se-GSHpx of 60% induced by copper (Cu) deficiency had no effect on GST activity or concentration. Increased GST activity in Se deficiency has previously been postulated to be a compensatory response to loss of Se-GSHpx, since some GSTs have a non-Se-glutathione peroxidase (non-Se-GSHpx) activity. However, the GST isoenzymes determined in this study, GST Yb1Yb1, GST YcYc and GST YaYa, are known to have up to 30-fold differences in non-Se-GSHpx activity, but they were all significantly increased to a similar extent in the Se-deficient rats.

Impaired hepatocellular integrity during general anaesthesia, as assessed by measurement of plasma glutathione S-transferase

The measurement of plasma glutathione S-transferase (GST) concentrations have been used to assess the changes in hepatocellular integrity which occur following general anaesthesia. Of 20 selected patients, who received halothane for minor urological procedures, 16 showed a small transient rise in GST between 1 h and 3 h after anaesthesia. Similar changes were also observed in 8 consecutive patients who received halothane for various operative procedures. In 3 of these 28 patients a marked secondary rise in plasma GST was observed 24 h after anaesthesia. No significant changes in ALT were observed in either of the groups of patients. These data indicate two possible phases of hepatotoxicity following halothane administration which results in a transient impairment in hepatocellular integrity in the majority of patients who undergo anaesthesia with this agent.