Substrate-induced modifications of the intrinsic fluorescence of the isolated adenine nucleotide carrier protein: demonstration of distinct conformational states.

The effects of ATP or ADP and the specific inhibitors carboxyatractyloside (CATR) and bongkrekic acid (BA) on the conformation of the isolated adenine nucleotide (AdN) carrier protein were studied by fluorescence spectroscopy. The addition of ATP to the AdN carrier resulted in a rapid fluorescence increase of the tryptophanyl residue(s) at 355 nm, which leveled up in less than 1 s at 22 degrees C. Among the natural nucleotides, only ATP and ADP were effective. At 10 degrees C or below, the kinetics of the fluorescence increase induced by ATP were biphasic, consisting of a rapid phase of less than 1 s, followed by a slower phase that lasted for a few seconds and had virtually the same amplitude as the rapid one. Both phases were abolished when CATR was added prior to ATP or fully reversed when CATR was added after the fluorescence response to ATP had been elicited. The number of CATR binding sites present on the carrier protein was determined by CATR specific inhibition of the ATP-induced increase in intrinsic fluorescence. The calculated number of CATR sites was equal to that found by another method based on the use of the same preparation of AdN carrier loaded with fluorescent nucleotide naphthoyl-ATP and on the CATR-induced release of the bound naphthoyl-ATP, demonstrating the reliability of the intrinsic fluorescence assay. Addition of BA prior to or together with ATP nearly doubled the amplitude of the ATP-induced fluorescence signal. At 10 degrees C or below, the fluorescence response to ATP in the presence of BA could also be decomposed into rapid and slow phases.(ABSTRACT TRUNCATED AT 250 WORDS)