Electron microscopy of negatively stained and freeze-etched high density lipoprotein-3 from human serum.

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Abstract
High density lipoproteins of d = 1.12 to 1.21 g/ml from human serum (HDL3) were studied by electron microscopy with both negative staining and freeze-etching techniques. For the negatively stained specimens, a modified conventional transmission electron microscope as well as a scanning transmission electron microscope were used. The freeze-etched specimens were examined by a conventional transmission electron microscope. The diameter of HDL3 was found to be 105 +/- 4 A by freeze-etching and 94 +/- 6 A by negative staining. The surface of the HDL3 particles exhibited about 12 discrete domains, 28 +/- 3 A (freeze-etched) and 28 +/- 4 A (negatively stained) in diameter, of undefined chemical composition. Moreover, the freeze-etched specimens revealed an inner core 40 +/- 2 A in diameter, corresponding to estimated values reported previously. All information is consistent with the HDL3 model proposed by B. W. Shen, F. J. Kézdy, and A. M. Scanu [(1977) Proc. Natl. Acad. Sci. USA 74, 837-841], with additional evidence for well-defined surface substructure. The consistency of the images obtained with the various electron microscopy techniques and the marked change in the appearance of the surface in the HDL3 preparations that were digested by phospholipase A2 (EC 3.1.1.4) support the validity of the interpretation.