Tissue distribution of a major mevalonate pyrophosphate decarboxylase in rats.

The 45- and 35-kDa subunits of mevalonate pyrophosphate decarboxylase (MPD) have been purified from rat liver. In this study, we examined the relationship between 45- and 35-kDa MPD and the tissue distribution of a major MPD in rat liver. When the crude extract of rat liver fed on normal chow was subjected to immunoblot analysis using anti-rat 45-kDa MPD antibody, only the 45-kDa band was detected. In a pulse-chase experiment using anti-rat 45-kDa MPD antibody, there was no precursor-product relationship between the 45- and the 35-kDa MPD. In immunoprecipitation, more than 85% of MPD activity in the rat liver was depleted from the crude extract with an excess of the above antibody. When 45-kDa MPD contents in tissues were analyzed by immunoblotting, a single protein band with an apparent molecular weight of 45 kDa was detected in all tissues. The specific protein content of 45-kDa MPD in liver was markedly higher than in other tissues. The activity/amount ratio varied among brain, liver, and testis, being significantly highest in the liver. From these data, it is suggested that 45-kDa MPD serves as a major enzyme involved in cholesterol biosynthesis in rat liver and that a tissue-specific regulator or isozyme of 45-kDa MPD is present in rat liver.

Mevalonate pyrophosphate decarboxylase is predominantly located in the cytosol of rat hepatocytes.

Mevalonate pyrophosphate decarboxylase (MPD) is found in the 100000 x g supernatant fraction of cells or tissues and is considered to be a cytosolic protein. Recently, other groups reported that MPD is mostly located in the peroxisomes. In this study, we used two different methods to determine whether MPD is predominantly located in the peroxisomes or the cytosol of rat hepatocytes. 1) In permeabilized rat hepatocytes or normal rat kidney cells treated with digitonin, which lack cytosolic enzyme, MPD was mainly present in the medium. 2) Double immunofluorescent labeling of cells with both anti-MPD antibody and anti-hexokinase antibody yielded an immunofluorescent pattern for both enzymes typical of the cytosolic protein. These results indicate that MPD is predominantly located in the cytosol of rat hepatocytes.

Difference in subcellular distribution between 45- and 37-kDa mevalonate pyrophosphate decarboxylase in rat liver.

We previously reported that the CP diet (a diet containing 5% cholestyramine and 0.1% pravastatin)-induced new species of 37-kDa mevalonate pyrophosphate decarboxylase (MPD) was characteristically and immunologically very similar to the well-known 45-kDa MPD. In the present study, we found a difference in subcellular distribution between 45- and 37-kDa MPD by cell fractionation and immunoblot analysis. The cytosol fraction contained 45- and 37-kDa MPD. Peroxisomal fraction contained a small amount of 45-kDa MPD, but not 37-kDa MPD. Also, 45-kDa MPD in peroxisome is localized in the matrix. From these data, the difference in subcellular distribution between 45- and 37-kDa MPD may be due to differences in the physiological role of cholesterol biosynthesis in rat liver.

Mevalonate pyrophosphate decarboxylase in stroke-prone spontaneously hypertensive rat is reduced from the age of two weeks.

We carried out a comparison of tissue distribution of mevalonate pyrophosphate decarboxylase (MPD) between normotensive Wistar Kyoto rat (WKY) and stroke-prone spontaneously hypertensive rat (SHRSP) using Western blotting. However, there was no difference in tissue distribution of MPD between WKY and SHRSP, expect in brain and liver. We then compared the MPD between WKY and SHRSP liver at several weeks of age. We found that MPD in the liver as well as brain of SHRSP was significantly reduced from two weeks of age. This data is useful to identify or understand the mechanism underlying the reduced amount of MPD in SHRSP.

Lower mevalonate pyrophosphate decarboxylase activity is caused by the reduced amount of enzyme in stroke-prone spontaneously hypertensive rat.

Spontaneously hypertensive rat (stroke-prone) (SHRSP) has a low serum cholesterol level as compared with the normotensive Wistar Kyoto rat (WKY). We previously indicated that the lower activity of mevalonate pyrophosphate decarboxylase (MPD) was responsible for the reduced cholesterol biosynthesis in the liver of SHRSP [Sawamura et al. (1992) J. Biol. Chem. 267, 6051-6055]. To elucidate the mechanism of the reduced activity, we purified liver MPD from SHRSP treated with cholestyramine and pravastatin in this study. We compared its enzymatic properties with those of the enzyme from WKY, and also measured the amounts of MPD in the crude extract of various tissues in WKY and SHRSP by Western blot analysis. Results indicated that (i) MPD of SHRSP has essentially the same properties as MPD of WKY, except for a difference in the dependency on divalent cations. (ii) The amount, as well as the activity, of MPD in the crude extract of brain and liver was reduced in SHRSP. (iii) There was no difference between SHRSP and WKY, in the ratio of the enzyme activity to the amount of MPD in the crude extract. These data led us to conclude that the lower activity of MPD was caused by the reduced amount of this enzyme in SHRSP.

Purification and characterization of two mevalonate pyrophosphate decarboxylases from rat liver: a novel molecular species of 37 kDa.

The biosynthesis of cholesterol is regulated mainly by HMG-CoA reductase, however, recent studies indicated the pivotal role of another enzyme in cholesterol homeostasis. A previous report showed a marked decrease in the activity of mevalonate pyrophosphate decarboxylase (MPD) in stroke-prone spontaneously hypertensive rats and its possible involvement in the pathogenesis of the disorder. In this study, we purified liver MPD from rats fed a diet containing cholestyramine and pravastatin (CP diet) using conventional chromatographic techniques. We obtained two electrophoretically homogeneous enzyme preparations; 45 and 37 kDa proteins with specific activities of 8.0 and 7.4 micromol/min/mg, respectively. The enzymes showed similar molecular weights of 90 kDa, as judged on gel permeation chromatography. A kinetic study indicated apparent Km values for mevalonate pyrophosphate and ATP of 22.7 microM and 0.71 mM, respectively, for the 45 kDa MPD, and 20.0 microM and 0.80 mM, respectively, for the 37 kDa MPD. Half maximum activities were observed at 1.5 mM and 1.1 mM Mg2+ for the 45 and 37 kDa MPDs, respectively. Both enzymes required ATP as a phosphate acceptor, and in addition Mg2+, Mn2+, and Co2+ were effective as divalent cations. The optimum pH for both enzymes was 7.0. The isoelectric points for the 45 and 37 kDa MPDs were 5.6 and 5.4, respectively. Polyclonal antiserum raised against the 45 kDa enzyme detected both the 45 and 37 kDa bands on immunoblots with CP diet-induced liver crude extract as an antigen. However, non-induced liver contained the 45 kDa protein but not the 37 kDa protein. These results indicated that the CP diet induced a new species, 37 kDa, of MPD which is characteristically and immunologically very similar to the well-known 45 kDa MPD.

Biosynthetic transport of a major lysosome-associated membrane glycoprotein 2, lamp-2: a significant fraction of newly synthesized lamp-2 is delivered to lysosomes by way of early endosomes.

Lysosomal membranes contain two highly glycosylated proteins, designated as lamp-1 and lamp-2, as major components. Lamp-1 and lamp-2 are similar to each other in the protein structure. Here, we investigated the biosynthetic transport of lamp-2 through the endocytic vacuoles in cultured rat hepatocytes in comparison with that of lamp-1, which has previously been studied [Akasaki et al. (1995) Exp. Cell Res. 220, 464-473]. Newly synthesized lamp-2 (NS-lamp-2) was transported to the trans-Golgi from rough endoplasmic reticulum with a half time (t1/2) of 32 min, more slowly than NS-lamp-1 (t1/2 = 13 min). After leaving the trans-Golgi, NS-lamp-2 is transferred to at least three compartments; the cell surface (t1/2 = 47 min), cell peripheral early endosomes (t1/2 = 38 min) and perinuclear late endosomes (t1/2 = 48 min). NS-lamp-2 transported to any compartment is delivered finally to lysosomes (t1/2 = 90 min). A significant fraction of NS-lamp-2 (45% of the total) was transported from the trans-Golgi to early endosomes, and then delivered to dense lysosomes via perinuclear late endosomes, whereas a major portion of NS-lamp-1 follows an intracellular route to late endosomes without passing through the cell periphery. NS-lamp-2 leaves the cell peripheral region more rapidly than NS-lamp-1. The kinetic and quantitative data for biosynthetic transport of NS-lamp-2 to early endosomes and the cell surface indicate that NS-lamp-2 may be transported first to early endosomes, from which a small portion of it (approximately 3.5% of the total) moves to the plasma membrane via a recycling system. In contrast, a small fraction of NS-lamp-1 is transported to the plasma membrane directly from the trans-Golgi, since NS-lamp-1 is delivered to the plasma membrane and early endosomes with almost the same half times.

Biosynthetic transport of a major lysosomal membrane glycoprotein, lamp-1: convergence of biosynthetic and endocytic pathways occurs at three distinctive points.

We studied the kinetics of the biosynthetic transport of lysosome-associated membrane glycoprotein-1 (lamp-1) to the endocytic compartments in cultured rat hepatocytes. Newly synthesized lamp-1 (NS-lamp-1) was transported to the trans-Golgi from rough endoplasmic reticulum with a half time (t1/2) of 13 min. From the trans-Golgi, at least 25% of NS-lamp-1 was delivered to the cell periphery: to the cell surface and early endosomes with t1/2 s of 32 and 33 min, respectively. A comparison of the kinetics of the biosynthetic transport of lamp-1 to both compartments demonstrated that NS-lamp-1 takes two peripheral routes from the Golgi apparatus; it is delivered to early endosomes directly and after reaching the cell surface. A major portion of NS-lamp-1 follows a direct intracellular pathway to late endosomes (t1/2 = 45 min) and subsequently to lysosomes (t1/2 = 85 min). The kinetic data of the biosynthetic transport to these endocytic vacuoles suggested that a significant fraction of NS-lamp-1 returns to the late endosomes immediately after its arrival at lysosomes and that there is a unique retrograde delivery of NS-lamp-1 from late to early endosomes prior to its transport to lysosomes. Thus, in cultured rat hepatocytes, the lamp-1 biosynthetic and the endocytic pathways converge at the three distinctive points. Late endosomes are centrally situated in the complex biosynthetic route of lamp-1.

Cycling of an 85-kDa lysosomal membrane glycoprotein between the cell surface and lysosomes in cultured rat hepatocytes.

We studied the endocytic transport of an 85-kDa lysosomal membrane glycoprotein (LGP85) from the cell surface to lysosomes in cultured rat hepatocytes. Fab' fragments of a monoclonal antibody against LGP85 (YA30 mAb) were conjugated with horseradish peroxidase (HRP) and then used as probes to monitor the endocytic transport of LGP85 from the plasma membrane to lysosomes. Continuous internalization and lysosomal transport of HRP-YA30 mAb Fab' occurred in the hepatic cells, resulting in its accumulation in the dense lysosomal fraction obtained from the cells on Percoll density centrifugation. The endocytic transport of HRP-YA30 mAb continued in the presence of the protein synthesis inhibitor, cycloheximide, indicating that LGP85 is cycled between the cell surface and lysosomes or endosomes, like other lysosomal membrane glycoproteins, lamp-1 and lamp-2, as reported previously [Akasaki et al. (1993) J. Biochem. 114, 598-604]. The half times (t1/2) of internalization and lysosomal transport of LGP85 were 32 min and 2.0 h, respectively. The kinetics of endocytic transport for LGP85 are very similar to those of lamp-1 and lamp-2. LGP85 possesses a short cytoplasmic tail whose amino acid sequence is quite different from those of lamp-1 and lamp-2. Therefore, these results suggested that continuous internalization from the cell surface and lysosomal transport of of endogenous LGP85 occur through a mechanism that can recognize this novel amino acid sequence, probably a Leu-Ile-containing motif, in normal hepatic cells of rat.