Delta'-Tetrahydrocannabinolic acid (THCA) synthase is the enzyme that catalyzes the oxidative cyclization of cannabigerolic acid into THCA, the acidic precursor of Delta'-tetrahydrocannabinol. We developed a novel expression system for THCA synthase using a methylotrophic yeast Pichia pastoris as a host. Under optimized conditions, the transgenic P. pastoris secreted similar to 1.32 nkat/l of THCA synthase activity. and the culture medium, from which the cells were removed, effectively synthesized THCA from cannabigerolic acid with a similar to 98% conversion rate. The secreted THCA synthase was readily purified to homogeneity. Interestingly, endoglycosidase treatment afforded a deglycosylated THCA synthase with more catalytic activity than that of the glycosylated form. The non-glycosylated THCA synthase should be suitable for Structure-function studies because it displayed much more activity than the previously reported native enzyme from Cannabis sativa as well as the recombinant enzyme from insect cell cultures. (c) 2007 Elsevier Inc. Ail rights reserved.