Human apolipoprotein B (apoB) mRNA: identification of two distinct apoB mRNAs, an mRNA with the apoB-100 sequence and an apoB mRNA containing a premature in-frame translational stop codon, in both liver and intestine.

Human apolipoprotein B (apoB) is present in plasma as two separate isoproteins, designated apoB-100 (512 kDa) and apoB-48 (250 kDa). ApoB is encoded by a single gene on chromosome 2, and a single nuclear mRNA is edited and processed into two separate apoB mRNAs. A 14.1-kilobase apoB mRNA codes for apoB-100, and the second mRNA, which codes for apoB-48, contains a premature stop codon generated by a single base substitution of cytosine to uracil at nucleotide 6538, which converts the translated CAA codon coding for the amino acid glutamine at residue 2153 in apoB-100 to a premature in-frame stop codon (UAA). Two 30-base synthetic oligonucleotides (nucleotides 6523-6552 of apoB mRNA), designated apoB-Stop and apoB-Gln, were synthesized containing the complementary sequence to the stop codon (UAA) and glutamine codon (CAA), respectively. Analysis of intestinal apoB mRNA by hybridization with apoB-Stop and apoB-Gln probes and sequence analysis of apoB clones in two independent human small intestinal cDNA libraries established that intestinal apoB mRNA contained both the apoB mRNA that codes for apoB-100 and the apoB mRNA containing the premature in-frame stop codon, which codes for apoB-48. Investigation of hepatic apoB mRNA and two hepatic cDNA libraries by hybridization with the apoB-Stop and apoB-Gln synthetic probes as well as by cDNA sequencing revealed that liver apoB mRNA also contains both the apoB-100 mRNA and the apoB-48 mRNA containing the stop codon. The combined results from these studies establish that both human intestine and liver contain the two distinct apoB mRNAs, an mRNA that codes for apoB-100 and an apoB mRNA that contains the premature stop codon, which codes for apoB-48. The premature in-frame stop codon is not tissue specific and is present in both human liver and intestine.