The alphastat hypothesis in respiratory control and acid-base balance

This selective review 1) evaluates recent interpretations that broaden the definition of the alphastat hypothesis, 2) proposes that central chemoreception and acid-base regulation via ion transport involve proteins conforming to the alphastat hypothesis, and 3) describes, using recent evidence, possible candidates for these proteins. The alphastat hypothesis states that proteins that contain appropriate function-determining titratable groups maintain a constant charge state and unaltered function with temperature-dependent pH changes but can be very sensitive to isothermal pH changes. Appropriate groups, e.g., imidazole histidine, are determined by the pK and the effect of temperature on the pK. The hypothesis explains how protein structure and function can be conserved among a diversity of vertebrate and invertebrate pH values. It also suggests a mechanism for sensing or regulating temperature-independent pH changes, e.g., in central chemosensitivity and transmembrane ion exchange. Possible candidates for such alphastat-conforming proteins include two, the glutamate receptor and the Na(+)-H+ antiporter, for which recent evidence indicates the presence of numerous histidines at probable function-determining sites and demonstrates pH sensitivity inhibitable by the histidine blocker diethylpyrocarbonate (DEPC).