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CALCIUM‐DEPENDENT BINDING OF BRAIN GLUTAMATE DECARBOXYLASE TO PHOSPHOLIPID VESICLES

1978; Wiley; Volume: 31; Issue: 5 Linguagem: Inglês

10.1111/j.1471-4159.1978.tb06244.x

1471-4159

Manuel Covarrubias, Ricardo Tapia,

Cellular transport and secretion

Abstract The binding of glutamate decarboxylase (GAD), to phospholipid vesicles (liposomes) in the absence and in the presence of several Ca 2+ and Mg 2+ concentrations was studied. Phosphatidylcho‐line‐phosphatidylserine (4:1) liposomes are capable of binding GAD in a Ca 2+ ‐dependent manner. The per cent of GAD bound increased from 5 to 65°., in a sigmoid shape with Ca 2+ concentrations in the 0.2‐4 m m range. Mg 2+ also induces GAD binding but is less effective than Ca 2+ The Ca 2+ ‐dependent binding of GAD is not the result of unspecific association of protein, since Ca 2+ did not promote any binding of choline acetyltransferase or lactate dehydrogenase. Furthermore, the relative specific activity ( o o enzyme activity/% protein) of GAD associated to liposomes increases 4‐fold from 0 to 2 m m Ca 2+ . The per cent of GAD bound attains a plateau at a ratio phospholipid/protein of about 1.5. and decreases when the pH increases from 6.5 or 6.8 to 7 or 7.25. Na + or K + at a 100m m concentration also induce binding of GAD to liposomes. Phosphatidylcholine liposomes (without phosphatidylserine) practically did not bind GAD at any Ca 2+ concentration. The Ca 2+ ‐dependent association of GAD to phosphatidylcholine‐phosphatidylserine liposomes is very similar to that previously reported using brain membranes, and it correlates also well with the reported Ca 2+ ‐dependent aggregation of phosphatidylserine molecules in phospholipid membranes of similar composition. It is concluded that phosphatidylserine is probably involved in the Ca 2+ ‐dependent binding of GAD to brain membranes. Phospholipid vesicles seem to be a useful experimental model for studying the mechanisms of this GAD association to membranes and the possible physiological implications of the GAD‐Ca 2+ ‐membrane interaction regarding the release of newly synthesized GABA from nerve endings.