Experiments with aspartate transcarbamylase: 1. Isolation of pyridoxal-P: catalytic subunit reduced complexes. Effects of allosteric and substrate ligands on fluorescent properties. 2. Synthesis and use of reactive transition state analogs. 3. Elucidation of the nature and significance of a second binding site for dicarboxylic acids in catalytic subunits. 4. Nuclear magnetic resonance experiment with dideuterio-L-malate and fluorinated derivatives of carbamyl-P and catalytic subunit. 5. Effect of phosphonacetyl-L-aspartate on the de novo pyrimidine pathway in vivo.