A Novel Strategy for Isolating Hepr, a Human Small Intestinal Cytidine Deaminase

Abstract

Apolipoprotein B (apo B)is a large hydrophobic protein which circulates in two distinct forms, each the product of a single apo B gene (Chan, 1992). The small intestine synthesizes and secretes a form of apo B referred to on a centile scale as apo B48, a protein of 264 KDa, which is associated with chylomicron remnants (Powell et al., 1987, Chen et al., 1987). Human liver produces a form of apo B referred to as apo B100. Apo B100, a 550 KDa protein, circulates in association with low density lipoproteins, the major transport vehicle for cholesterol ester in humans (Chan, 1992). This organ- specific production of apo B isophorms has as its basis a unique post- transcriptional modification to the apo B mRNA. Apo B transcription occurs in the liver and small intestine and yields a 24 Kb mRNA species which contains a CAA codon at position 2153, encoding glutamine in apo B 100 and which is specified in the genomic sequence. In the small intestine, this apo B mRNA undergoes a site-specific cytidine deamination which results in the production of a UAA codon and thereby specifies an in-frame translational stop (Powell et al., 1987, Chen et al., 1987). Work conducted over the last few years using isolated intestinal cell extracts and synthetic apo B RNA templates has enabled investigators to determine that apo B mRNA editing is a process mediated by protein(s) and with the characteristics of an enzymatic reaction (Teng and Davidson, 1992).