Photoregulation of an enzymic process by means of a light-sensitive ligand.

A specific inactivator of chymotrypsin, p-azophenyldiphenylcarbamyl chloride, exists as two geometric isomers, cis and trans, which are interconvertible by means of light. The cis-isomer is five times more reactive than the more stable trans-isomer, and is obtained by exposure of the latter to light of 320 nanometer wavelength. The trans-isomer can be regained by exposure of the cis-isomer to light of 420 nanometer wavelength. This interconversion can be made to occur in aqueous solution in the presence of the enzyme under conditions in which the trans-isomer reacts relatively slowly with chymotrypsin. Thus, it is possible to regulate the rate of inactivation of chymotrypsin by using light of the appropriate wavelength. This system is presented as a model for some of the light-sensitive metabolic systems present in living organisms.