Characterization of lecithin:Cholesterol acyltransferase from human plasma: II. Physical properties of the enzyme

The physical properties of purified human plasma lecithin:cholesterol acyltransferase (LCAT) were investigated by techniques including analytical ultracentrifugation, ultraviolet spectroscopy, electrofocusing, and circular dichroism. The partial specific volume of LCAT was determined by sedimentation equilibrium ultracentrifugation experiments in H₂O and D₂O solutions (0.702 ml/g). The M_r was 67,000 by sodium dodecyl sulfate (SDS)-gel electrophoresis and 60,000 by sedimentation equilibrium ultracentrifugation. The discrepancy between the two sets of data presumably arose from the glycoprotein nature of the enzyme. Studies of the ultraviolet spectrum indicated that LCAT contained 6.5% ($\text{w}\text{w}$) tyrosine which corresponds to approximately 18 tyrosine residues/mol of LCAT (polypeptide M_r 45,000). Spectrophotometric titration of the ionizable phenolic side chains indicated that nearly all the tyrosine residues were buried at neutral pH while they became gradually exposed at higher pH. The apparent pK of this transition was about 12.0 contrasted with 9.8, the apparent pK of ionization of the free tyrosyl groups.     

RNA processing in prokaryotic cells

RNA processing in Escherichia coli and some of its phages is reviewed here, with primary emphasis on rRNA and tRNA processing. Three enzymes, RNase III, RNase E and RNase P are responsible for most of the primary endonucleolytic RNA processing events. The first two are proteins, while RNase P is a ribozyme. These three enzymes have unique functions and in their absence, the cleavage events they catalyze are not performed. On the other hand a relatively large number of exonucleases participate in the trimming of the 3′ ends of tRNA precursor molecules and they can substitute for each other. Primary processing is the first event that happens to the nascent RNA molecule, while in secondary RNA processing, the substrate is a product of a primary processing event. Although most RNA processing occurs in RNP particles, it seems that only in secondary RNA processing is the RNP particle required for the reaction. Bacteria and especially bacteriophages contain self-splicing introns which in cases were probably acquired from other species.