THE EFFECT OF DICYCLOHEXYLCARBODIIMIDE AND CYCLOPIAZONIC ACID ON THE DIFFERENCE FTIR-SPECTRA OF SARCOPLASMIC-RETICULUM INDUCED BY PHOTOLYSIS OF CAGED-ATP AND CAGED-CA2+

The photochemical release of Ca2+ from caged-Ca2+ in the absence of ATP, and the release of ATP from caged-ATP in the presence of Ca2+ induce characteristic difference FTIR spectra on rabbit sarcoplasmic reticulum that are related to the formation of Ca2-E1 and E approximately P intermediates of the Ca2+-ATPase, respectively. Dicyclohexylcarbodiimide (10 nmol/mg protein) abolished both the Ca2+-and ATP-induced difference FTIR spectra parallel with inhibition of ATPase activity. Cyclopiazonic acid (50 nmol/mg protein) inhibited the Ca2+-induced difference spectrum measured in the absence of ATP, but had no significant effect on the ATP-induced difference spectrum measured in the presence of 1 mM Ca2+. The dog kidney Na+,K+-ATPase did not give significant difference spectrum after photolysis of caged-ATP in Ca2+-free media containing 90 mM Na+ and 10 mM K+, with or without ouabain. We propose that both the Ca2+ and the ATP-induced difference FTIR spectra of the Ca2+-ATPase reflect the occupancy of the high-affinity Ca2+ transport site of the enzyme.