Bovine liver glutamate dehydrogenase is known to bind reduced coenzyme at two sites/subunit, one catalytic and one regulatory; ADP competes for the latter site. The enzyme is here shown to be catalytically active with the thionicotinamide analogue of NADPH ([SINADPH) [7], and calf brain tubulin [8]. Energy transfer between two chromophores is possible as long as the emission spectrum of the energy donor overlaps the absorption spectrum of the energy acceptor. When combined with the spectral properties of the donor and acceptor, the amount of quenching of the donor's fluorescence by the acceptor leads to information about the distance between the chromophores.The activity of bovine liver glutamate dehydrogenase is affected by purine nucleotides. Either NADPH or NADH is capable of binding to the catalytic site [9] and high concentrations of these coenzymes inhibit the enzyme by binding to a distinct reduced coenzyme regulatory site [lo]. ADP, which activates the enzyme, has two binding sites per subunit [Ill.