3-hydroxy-3-methylglutaryl coenzyme A lyase: targeting and processing in peroxisomes and mitochondria

3-Hydroxy-3-methylglutaryl coenzyme A lyase (HL, E.C. 4.1.3.4) has a unique dual localization in both mitochondria and peroxisomes, Mitochondrial HL ( similar to 31.0 kDa) catalyzes the last step of ketogenesis; the function o f peroxisomal HL (similar to 33.5 kDa) is unknown. On density gradient frac tionation, normal human lymphoblasts contain both peroxisomal and mitochond rial HL whereas in lymphoblasts from a patient with Zellweger syndrome, in which functional peroxisomes are absent, only the mitochondrial HL isoform was present. To study the kinetics of the dual targeting of HL, we performe d pulse-chase experiments in normal and Zellweger cells, Pulse-chase studie s revealed a biphasic curve for processing of the HL precursor. The first p hase, with a calculated half-life of similar to 3 h in both normal and Zell weger fibroblasts and lymphoblasts and in HepG2 cells, presumably reflects mitochondrial import and processing of the precursor; the second (t(1/2), 1 2-19 h) is present only in normal cells and presumably represents the half- life of peroxisomal HL. The half-life of mature mitochondrial HL was 14 to 19 h in both normal and Zellweger cells. Studies of the HMG-CoA lyase precu rsor in isolated rat mitochondria showed a rate of processing similar to 2. 6-fold lower than that of the ornithine transcarbamylase precursor.