Molecular dimensions of the SS and FF phenotypes of human placental alkaline phosphatase. Rotary shadowing and negative staining electron microscope measurements.

The two most common homologous phenotypes (SS and FF) of human placental alkaline phosphatase were purified and observed in the electron microscope by rotary shadowing and negative staining techniques. In the rotary shadowing technique, the molecules of the two phenotypes appeared to be approximately elliptical with slit-like structures in the center of the molecules, suggestive of the groove between two subunits. The dimensions of the rotary-shadowed molecules were calculated as 10.1 nm X 5.7 nm for SS and 10.1 nm X 5.6 nm for FF phenotypes. The negative staining technique delivered more fine detail of the molecules than rotary shadowing. The predominant shape of the molecules in this method appeared to be rectangular, with a longitudinal stain-filled groove and with each of the half molecules (presumably 65,000 Mr subunit) very often appearing bi-lobed. This accounts for the molecules which appear to have four pronounced electron-transparent regions. The dimensions of the negatively stained rectangular-shaped molecules were measured as 7.5 nm X 5.5 nm for SS and 7.6 nm X 5.4 nm for FF phenotypes. No significant difference in electron microscopic appearance between the SS and FF phenotypes were observed.