Analytical subcellular fractionation of rat pituitary homogenates with special reference to the subcellular localization and properties of alkaline phosphatases.

Alkaline phosphatase activities of the virgin rat anterior pituitary were studied with a highly sensitive fluorometric assay. Tissue whole homogenates were fractionated on sucrose density gradients in a Beaufay automatic zonal rotor and the gradient fractions assayed for alkaline phosphatase, prolactin and various organelle marker enzymes. Alkaline phosphatase was distributed between two peaks on the gradient. The low-density (1.10-1.15 g . cm-3) alkaline phosphatase component co-sedimented with the plasma membrane marker, 5'-nucleotidase, had an apparent Km for 4-methylumbelliferyl phosphate of approx. 59 microM, and was inhibited by levamisole. The high-density (1.20-1.25 g . cm-3) peak was resistant to levamisole-inhibition, had an apparent Km of approx. 30 microM and its distribution was distinct from plasma membrane, Golgi, lysosome, endoplasmic reticulum, mitochondria and prolactin granule markers on the isopycnic gradients.