Rabbit hepatic lipase cDNA sequence: low activity is associated with low messenger RNA levels

We have investigated a possible mechanism for the reported low activity of hepatic lipase (HL) in the rabbit by cloning and sequencing the cDNA for rabbit HL and using the clone to quantify mRNA levels. A 1.6 kb cDNA clone was sequenced and found to encode the mature protein of 477 amino acids and 20 amino acids of the hydrophobic leader peptide. A high degree of amino acid sequence identity was demonstrated with human (81%) and rat (79%) HL. The putative active site was well conserved, and mutations reported to reduce activity in HL or lipoprotein lipase were not present in the rabbit sequence. The activity and mRNA levels were compared with those of the rat, an animal possessing relatively high HL activity. In post-heparin plasma of the rat, HL activity was nine times greater than in that of the rabbit (24.9 +/- 1.6 units per ml plasma, n = 5 vs. 2.7 +/- 0.1, n = 5, P = 0.0001). Comparison of mRNA levels was made by dot blot analysis of liver poly (A+) RNA obtained from each species and probed with either rabbit or rat HL cDNA, labeled to the same specific radioactivity. Specific HL mRNA levels were found to be nine times greater in the rat than in the rabbit (8.90 +/- 0.11 units, n = 5 vs. 1.00 +/- 0.01, n = 5, P = 0.0001). Thus, low hepatic lipase activity in the rabbit is associated with low mRNA levels, suggesting that the observed species difference in activity is due to differences in the level of mRNA.