Quercetin inhibits the sulfation of r(-)-apomorphine in human brain

The first aim of this investigation was to study the sulfation of R(-)-apomorphine in human brain. The second aim was to investigate the inhibition of R(-)-apomorphine sulfation by quercetin in human brain. R(-)-apomorphine is hereafter referred to as apomorphine. Apomorphine sulfation was measured in 5 brain specimens; 3 derived from the frontal cortex and 2 derived from the temporal cortex. The rate of apomorphine sulfation was 5.6 +/- 4.3 pmol/min/mg. The activities of SULT1A1 and SULT1A3, which were also measured in these samples, were 11 +/- 9.1 and 2.6 +/- 1.7 pmol/min/mg, respectively. The rate of apomorphine sulfation correlated with the activity of SULT1A1 (r = 0.989; p = 0.002) and SULT1A3 (r = 0.973; p = 0.005). Apomorphine sulfotransferase followed Michaelis-Menten kinetics, the Km (mean +/- SD) and Vmax values (mean +/- SD) of which, measured in 5 brain samples, were 32 +/- 7.3 microM and 8.9 +/- 7.9 pmol/min/mg, respectively. Quercetin was a potent inhibitor of apomorphine sulfation with an IC50 value, measured in 5 brain samples, of 16 +/- 2.3 nM. The inhibition mechanism of quercetin using apomorphine sulfation in 5 brain samples was mixed, non-competitive with a Ki and Kies (mean +/- SD) of 16 +/- 4.1 and 87 +/- 37 nM, respectively (p = 0.008). The intrinsic clearance value of apomorphine (mean +/- SD) was 247 +/- 170 ml/min/mg(-1) and was decreased to 100 +/- 85 ml/min/mg(-1) (p < 0.01) in the presence of 25 nM quercetin. In conclusion, apomorphine is sulfated in human brain. Sulfation might reduce the level of apomorphine in human brain and be a factor limiting the effect of this drug. Quercetin is a potent inhibitor of apomorphine sulfation and may inhibit the sulfation of apomorphine in human brain in vivo.