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Abstract— The sulpho-conjugates of vanilylmandelic acid (VMA) and homovanillic acid (HVA) were biosynthesized in vitro from ATP and sodium sulphate and from 3′-phosphoadenosine-5′-phosphosulphate (PAPS) obtained commercially or generated in vitro. Hydrolysis with sulphatase indicated the sulphate nature of these conjugates. In decreasing order of activity, the liver, small intestine and brain of various animals exhibited this sulpho-conjugatory ability. The Km, values of VMA and HVA for the sulphotransferase of mouse liver were, respectively, 1740 and 93 μM. Competition studies suggested that the same enzyme may be involved in the sulphation of these two acids.