Characterization of the submandibular gland microsomal calcium transport system

Calcium accumulation by submandibular gland microsomes (first described by Selinger and Naim, ((1970) Biochim. Biophys. Acta 323, 337-341) has been further characterized. Accumulation was concentration dependent, had a Km of 25 microM added calcium and a Vmax of 12 nM calcium/mg protein per min. No accumulation was observed in the presence of either the calcium ionophore A23187, or the detergent Triton X-100 (0.05). The divalent cations Sr2+ and Mn2+ inhibited accumulation competitively with Ki values of 67 microM and 200 microM, respectively. The effect of various enzyme inhibitors were tested on the microsomal calcium transport system and it was found that chlorpromazine, trifluoperazine, and DIDS all inhibited. The mitochondrial transport inhibitors ruthenium red and CCCP had no effect on transport. Experiments directed at clarifying the cellular location of the system are described. It was found that the membrane vesicles responsible for transport show different purification properties than the membrane vesicles which contain the standard enzyme markers for total and rough endoplasmic reticulum, Golgi apparatus, plasma membrane, and lysosomes. These conclusions are based upon experiments using these properties for membrane purification, density, size, and electrophoretic mobility. Three possible explanations of the results are given and it is organelles. The significance of the results in: (1) understanding the biochemical properties of the submandibular gland microsomal calcium transport system, (2) clarifying the cellular location of the system, and (3) clarifying the function of the system in salivary secretion are discussed.