Prospective Inhibitors of Serine Hydroxymethyltransferase

Abstract

The biochemical role of serine and serine hydroxymethy!transferase (SHMT) are discussed, and the significance of this enzyme in tumour cells is reviewed. The activities of SHMT were examined in various tumour cell lines and in a variety of murine organs. The enzyme was partially purified from murine L1210 leukaemia cells. DL-α-Vinyl-, DL-α-allyl- and DL-α-propargy] serine were synthesised as potential irreversible enzyme activated inhibitors of SHMT. The preparative routes involved alkylations of appropriate stabilised anions. The testing of the o-substituted compounds against total K562 cell SHMT showed DL-α-vinylserine to be a competitive inhibitor, with a Ki of 15.20 mM. DL-α-Allyl- and DL-α-propargylserine caused no inhibition of the enzyme from this source. No dehydroxymethylation of the α-substituted compounds could be detected using the partially purified L1210 SHMT enzyme. The compounds were also not dehydroxymethylated by the homogeneous rabbit liver enzyme, nor was there any evidence for quinonoid formation, or any change in the absorbance of the enzyme. The crystal structure of DL-α-vinylserine was determined by direct methods. Molecular modelling studies showed that for a modelled α-vinylserine-PLP conjugate there is no reason why the correct orientation for α-β bond cleavage could not be achieved. Circular dichroism spectroscopy indicated that the lack of activity of these compounds was due to them not binding to the active site PLP in the enzyme from homogeneous rabbit liver. The compounds were shown not to have antibacterial properties, nor did they cause growth inhibition in the K562 or GM0621 human leukaemia cell lines. In conclusion, the α-substituted serine analogues were not effective inhibitors of SHMT.