Functional size analysis of pyrophosphatase from Rhodospirillum rubrum determined by radiation inactivation.

A radiation inactivation technique was employed to determine the functional size of pyrophosphatase (PPase) from the chromatophores of Rhodospirillum rubrum. The activities of hydrolysis and synthesis reactions of pyrophosphatase and its coupled proton translocation decayed in a simple exponential function with the increase of radiation dosages. D37 values of 5.2 +/- 0.7 and 5.8 +/- 0.8 Mrads were obtained for pyrophosphate hydrolysis and its associated proton translocation yielding molecular masses of 167.7 +/- 30.7 and 156.3 +/- 26.6 kDa, respectively. Similarly, a D37 value of 4.4 +/- 0.6 Mrads was measured for the acid-base induced pyrophosphate synthesis resulting in a radiation sensitive size of 196.3 +/- 31.9 kDa.