Ultrastructural localization of brain 'vitamin D-dependent' calcium binding proteins

Rat brain vitamin D-dependent calcium-binding protein (D-CaBP) was assessed for vitamin D dependency, calcium binding and ultrastructural localization within neurons. No evidence of vitamin D dependency could be derived from the experiments on vitamin D-deficient rats. A 95% pure extract of the 27-kDa brain D-CaBP was shown to bind 45Ca on nitrocellulose membrane after sodium dodecyl sulphate-electrophoresis, specifically on the 27-kDa CaBP band. Immunogold staining with electron microscopy allowed detection of D-CaBP into Purkinje cells and climbing fibers of the cerebellum. The immunoreactivity was found to be hyaloplasmic and never membrane-bound. It was present in neuronal soma, neurites and postsynaptic as well as presynaptic terminals. These findings rule out D-CaBP as a possible neurotransmitter and bring further support to the hypothesis that the protein functions as a cytosolic calcium buffer. Immunohistochemical detection of D-CaBP is proposed as a means for morphologic detection of neurons with high calcium metabolism.