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Sulfotransferase (SULT) 1A1 Polymorphic Variants ^*1, ^*2, and ^*3 Are Associated with Altered Enzymatic Activity, Cellular Phenotype, and Protein Degradation

Fox Chase Cancer Center, Philadelphia, Pennsylvania

The superfamily of sulfotransferase (SULT) enzymes catalyzes the sulfate conjugation of several pharmacologically important endo- and xenobiotics. SULT1A1 catalyzes the sulfation of small planar phenols such as neurotransmitters, steroid hormones, acetaminophen, and p-nitrophenol (PNP). Genetic polymorphisms in the human SULT1A1 gene define three alleles, SULT1A1^*1, ^*2, and ^*3. The enzyme activities of the SULT1A1 allozymes were studied with a variety of substrates, including PNP, 17-estradiol, 2-methoxyestradiol, catecholestrogens, the antiestrogen 4-hydroxytamoxifen (OHT), and dietary flavonoids. Using purified recombinant SULT1A1 protein, marked differences in ^*1, ^*2, and ^*3 activity toward every substrate studied were noted. Substrate inhibition was observed for most substrates. In general, the trend in V_max estimates was ^*1 > ^*3 > ^*2; however, V_max/K_m estimate trends varied with substrate. In MCF-7 cells stably expressing either SULT1A1^*1 or ^*2, the antiestrogenic response to OHT was found to be allele-specific: the cells expressing ^*2 exhibited a better antiproliferative response. The intracellular stability of the ^*1 and ^*2 allozymes was examined in insect as well as mammalian cells. The SULT1A1^*2 protein had a shorter half-life than the ^*1 protein. In addition, the ^*2 protein was ubiquitinated to a greater extent than ^*1, suggesting increased degradation via a proteasome pathway. The results of this study suggest marked differences in activity of polymorphic SULT1A1 variants, including SULT1A1^*3, toward a variety of substrates. These differences are potentially critical for understanding interindividual variability in drug response and toxicity, as well as cancer risk and incidence.

Address correspondence to: Rebecca L. Blanchard, Merck and Co., Inc., 5 Sentry Parkway, BLB-33, Blue Bell, PA 19422. E-mail: rebecca_blanchard{at}merck.com

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