Properties of the pH-sensitive Site that Controls the Am= of Limulus Metarhodopsin

A B S X R A C T A pH-sensitive site controls the Amax of Limulus metarhodopsin. The properties of this site were examined using intracellular recordings of the early receptor potential (ERP) as a pigment assay. ERPs recorded over a range of extracellular pHs indicate that the apparent pK of the site is in the range of 8.3-8.6. Several lines of evidence indicate that the site responds directly to changes in extracellular pH (pHo) rather than to changes in intracellular pH (pHi) that follow as a secondary result of changing pHo: (a) the effect of changing pHo was rapid (<60 s); (b) when pHo was raised, the simultaneous rise in pHi, as measured with phenol red, was relatively small; (c) raising pHi by intracellular injection ofpH 10 glycine buffer did not affect the site; and (d) the effect of changing pHo could not be blocked by increasing the intracellular pH buffering capacity. It is concluded that the pH-sensitive site on metarhodopsin is on the extracellular surface of the plasma membrane.