We propose to determine the mechanism of action of the enzyme orotidine-5'-phosphate decarboxylase, and enzyme essential to the biosynthesis of the pyrimidine nucleotide. To this end, we intend to prepare pure orotidine-5'-phosphate decarboxylase in quantity by methods developed earlier under this grant, and attempt to crystallize the tight complex of this enxyme with 1-(5'-phospho-Beta-D-ribofuranosyl) barbituric acid. Professor W. N. Lipscomb has expressed interest in carrying out the X-ray crystallography of this complex; the X-ray information, in conjunction with data from our prior work, should permit us to arrive at the mechanism. In addition, we plan to continue our research directed to determining the mechanism of action of CTP synthetase. In particular, we wish to determine whether, in the absence of either ammonia of glutamine, the enzyme will catalyze the scrambling of the oxygen atoms of ATP. If the enzyme functions by way of transfer of monomeric metaphosphate ion to the carbonyl of UTP at position 2, such scrambling should occur.