The relationship between ATP and an electrogenic pump in the plasma membrane of Neurospora crassa.

Abstract

Sudden respiratory blockade has been used to study rapid changes of the resting membrane potential, of intracellular adenosine 5′-triphosphate (ATP) levels, and of pyridine nucleotide reduction inNeurospora crassa. Membrane depolarization occurs with a first-order rate constant of 0.167 sec−1, following a lag period of about 4 sec, at 24°C (ambient temperature). This depolarization is several-fold too slow to be directly linked to electron transfer, as judged from the rate of pyridine nucleotide reduction, but has essentially the same rate constant as the decay of ATP. The latter process, however, shows no lag period after the respiratory inhibitor is introduced. Plots of membrane potential versus the intracellular ATP concentration yield saturation curves which are readily fitted by a Michaelis equation, to which is added a constant term representing the diffusion component of membrane potential. Parameters obtained from such fits indicate the maximal voltage which the pump can develop at high ATP levels to be 300 to 350 mV, with an apparentK 1/2 of 2.0mm. The data strongly suggest that an electrogenic ion pump in the plasma membrane ofNeurospora is fueled by ATP; comparison of the measured membrane potentials with the energy available from hydrolysis of ATP indicates that two ions could be pumped for each molecule of ATP split.