Soluble & membrane-bound superoxide dismutases in a blue-green algae (Spirulina)and spinach

Adenylosuccinate synthetase (E.C. 6.3.4.4) $\text{Mg}{}^{\mathrm{++}}$ was assayed spectrophotometrically at 280 nm using two double-beam instruments with different stray light specifications, and by a radioactive assay. Using the spectrophotometric assay, inhibition by high concentration of GTP and by GMP is observed; a Hill plot of the GMP inhibition data obtained with the instrument of higher stray light seems to show cooperativity in GMP binding. The radioactive assay shows no inhibition by GTP at the concentrations studied and no cooperativity in the binding of GMP. This example illustrates the care that must be exercised in the interpretation of results obtained spectrophotometrically in solution of high absorbance.