Biosynthesis of the Purines

Purified formylglycinamide ribonucleotide amidotransferase polymerizes when stored in solution at 0°. This polymerization can be inhibited by the presence of 2-mercapto-ethanol or dithiothreitol and is therefore probably a result of the formation of intermolecular disulfide bridges. Although the glutamine-binding site of the enzyme contains at least one essential highly reactive, sulfhydryl group, this group is not involved in the formation of the intermolecular disulfide bridges, and the polymers retain full enzymatic activity. Preparations in an early stage of polymerization can be completely reduced to monomeric species by dithiothreitol or mercaptoethanol. Reduction of highly polymerized preparations is complete in the presence of sodium dodecyl sulfate and thiol reagent. Treatment of the enzyme with these reagents does not result in the breakdown of the enzyme into units smaller than the catalytic unit (monomer) of molecular weight, 133,000.