Aspartate aminotransferase and glutamate dehydrogenase activities in the squid giant nerve

The present study sought to investigate the presence and distribution of some enzymatic activities involved in the metabolism of glutamate in the giant nerve fiber of the tropical squid Sepioteuthis sepioidea. Specific activities of aspartate aminotransferase and glutamate dehydrogenase were evaluated in homogenates of the isolated giant fiber, extruded axoplasm, and axoplasm-free giant nerve fiber sheaths. The activities of both enzymes were present in the tissue. The specific activity of aspartate aminotransferase was similar in axoplasm and sheaths. However, the specific activity of glutamate dehydrogenase was an order of magnitude higher in the sheaths. This finding is discussed in the framework of the hypothesis that proposes that a differential distribution of the enzymes of the glutamatergic system between the axonal and neuroglial compartments forms part of a system of communication between these cells whose neuronal signal may be glutamate.