Is there a high molecular weight glutamic acid decarboxylase?

Gel-filtration chromatography from crude extracts of mouse brain indicates the presence of a high molecular weight (HMW) (more than 750 kd) and a low molecular weight (LMW) (87.5 Kd) glutamic acid decarboxylase (GAD) when they are concentrated prior chromatography either by precipitation with salts or ethanol. Kinetically both peaks of GAD-activity exhibit an almost identical Km for L-glutamate. Only LMW-GAD appears if the concentration step is carried out by ultrafiltration or if the extract is chromatographied without the concentrating step. HMW-GAD disappears from the chromatographic profiles if ethanolic extracts of GAD are treated with Triton X-100 before the chromatographic procedure. The sodium sulfate precipitation of a previously separated LMW-GAD gives rise to the reappearance of a HMW-GAD peak. Apparently HMW-GAD does not exist as a different molecular entity; indeed it may be an artefactual aggregation of LMW-GAD.