ELECTRON CRYOMICROSCOPY AND DIGITAL IMAGE-PROCESSING OF LIPOPROTEIN(A)

Electron cryomicroscopy was used to study the structure of human lipop rotein(a) (Lp(a)), a plasma lipoprotein implicated in cardiovascular d isease. An individual Lp(a) particle consists of a neutral lipid core within a shell of phospholipid, cholesterol and glycoprotein. In princ iple, electron cryomicroscopy images of single particles should contai n structural detail attributable to the density differences among thes e components and the surrounding buffer. We observed such structural d etail in images of frozen, hydrated Lp(a) particles. Lp(a) particles a ppeared to be roughly spherical in shape with an average diameter of 2 10 Angstrom. As is generally true for unstained samples in vitreous ic e, imaged with a low electron dose, these images have low contrast wit h low signal-to-noise ratios. To increase the signal-to-noise ratio, w e averaged classes of similar particles. We began with a set of 5813 r andomly oriented Lp(a) particles and generated classes using a linear multivariate statistical method, followed by hierarchical ascendent cl assification. Our initial classification, based on only the first eigh t eigenvectors, separated particles on the basis of gross size and sha pe. After a rough reference-free alignment step, a second classificati on used the finer details in the images. This approach yielded class a verages with structural detail only faintly visible in the raw, single images.