The effect of albumin on the metabolism of ethoxyresorufin through O-deethylation and sulphate-conjugation using isolated rat hepatocytes

Ethoxyresorufin was metabolised by suspensions of isolated rat hepatocytes through O-deethylation to resorufin, followed by sulphate-conjugation of the resorufin. The deethylation but not the conjugation was greatly induced by 3-methylcholanthrene pretreatment in vivo. Induction altered the apparent deethylation V_max but not the apparent K_m value. With control hepatocytes there was a 3-fold difference between the apparent V_max values for deethylation (0.07) and conjugation (0.22 nmole/min/10⁶ hepatocytes) but none between the apparent K_m values (1.3 μM, deethylation and 1.0 μM, conjugation). After induction the deethylation V_max (10 nmole/min/10⁶ hepatocytes) was almost 13-fold higher than the conjugation V_max (0.81 nmole/min/10⁶ hepatocytes), but again there was no difference in the K_m values for the two reactions (2 μM). A significant proportion of the deethylation product, resorufin, passed out from the hepatocytes and then re-entered them in order to undergo conjugation. Extracellular bovine serum albumin inhibited the conjugation by binding resorufin that had left the hepatocytes. Albumin greatly increased the total resorufin formed from ethoxyresorufin, despite inhibiting very slightly the initial rate of deethylation.