Presence of a highly efficient "binding" to bacterial contamination can distort data from binding studies

[3H]GABA at low concentrations (5-10 nM) was bound by what appeared to be a "GABA receptor binding site" in bacterial contamination originating from a batch of distilled water. Under experimental conditions similar to those usually employed in [3H]GABA binding studies, the apparent binding displayed a very high "specific" component and a high efficiency in terms of [3H]GABA bound per mg of protein. The "binding" was blocked by muscimol but not by isoguvacine, SR95531 and nipecotic acid. These characteristics suggest that the presence of such spurious binding in the experiments using 3H-labeled ligands in brain homogenates may not always be very obvious and, moreover, it can result in subtle, but serious, distortions of data from such studies, which may not be immediately recognized.