In Diethylpyrocarbonate-inhibited Intravesicular Acidification Was Specifically Achieved in the Presence of Extravesicular Glycyl-l-proline (gly-l-pro) as Well as of Glycyl-l-alanine (gly-l-ala) and D-phenylalanyl-l-alanine (d-phe-l-ala). H + /gly-l-pro Cotransport Displayed Saturable Kinetics, Invo