N-sulphation of desipramine in the rat brain

1. Amine N-sulphotransferase (NST) activity with desipramine (DMI) as substrate was assayed in vitro in various areas of the rat brain. Biosynthesis of 3'-phosphoadenosine-5'-phospho35sulphate (PAPS) from sodium 35sulphate and ATP was also measured by coupling it to the sulphation of minoxidil by minoxidil sulphotransferase (MST). 2. For the DMI-NST reaction, an apparent Km = 0.5 mM was obtained for DMI and two apparent Kms = 0.3 and 1.7 microM for PAPS, whereas in the PAPS-generating reactions, Km for sodium 35sulphate = 20 microM. 3. Both the enzyme activities were widely distributed in rat brain. The rate of NST activity was 2-3 orders of magnitude lower than that of PAPS generation. N-sulphoconjugation of DMI, which is proposed as a possible biotransformation pathway of DMI in the rat brain, could conceivably be supported adequately by the 'active sulphate' generated within the same areas of the brain.