TY - JOUR
T1 - Kinetic evidence for compartmentalization of myo-inositol in hepatocytes
AU - Sigal, Samuel Harold
AU - Yandrasitz, John R.
AU - Berry, Gerard T.
N1 - Funding Information:
From the Division of Biochemical Development and Molecular Diseases, The Children’s Hospital of Philadelphia; and the Department of Pediatrics, University of Pennsylvania School of Medicine, Philadelphia, PA. Submitted December 271991; acceptedApril 10, 1992. Supported by National Institutes of Health research Grants No. HD 08536 and HD 07107. Dr Sigal was a resident in training in the Department of Pathologv and Laboratory Medicine of the University of Pennsylvania School of Medicine. Address reprint requests to Gerard T Berry, MD, Division of Biochemical Development and Molecular Diseases, The Children’s Hospital of Philadelphia, One Children’s Center, 34th St and Civic Center Blvd, Philadelphia, PA 19104. Copyright 6 1993 by W B. Saunders Company 00260495/93/4203-0021$03.00/O
PY - 1993/3
Y1 - 1993/3
N2 - myo-Inositol uptake and conversion to phosphatidylinositol (PI) was studied in isolated rat hepatocytes. Uptake of myo-[2-3H]-inositol into the trichloroacetic acid (TCA)-soluble fraction showed no evidence of saturation, while incorporation into lipid had an apparent Km of 0.28 mmol/L for external myo-inositol. With 50 μmol/L myo-[2-3H]-inositol, approximately half of the radiolabel was found in lipid at 30 minutes. Glucose and galactose were weak inhibitors, while phlorizin at 1 mmol/L reduced uptake by 50%. Metabolic inhibitors reduced incorporation of myo-[2-3H]-inositol into lipid, but had no effect on uptake. Hepatocytes maintained myo-inositol levels of 0.4 mmol/L for 60 minutes when incubated with 50 μmol/L myo-inositol, but levels increased when incubated with 1 mmol/L myo-inositol. Efflux of label was studied in hepatocytes prelabeled for 20 minutes with myo-[2-3H]-inositol. Loss of label was initially rapid, but had slowed by 20 minutes, with much of the label remaining in the cells. Phlorizin inhibited the loss of myo-[2-3H]-inositol, while increasing myo-inositol concentration in the medium enhanced efflux. The effects of these agents on the rate of efflux was found in lipid rather than in the TCA-soluble myo-inositol fraction. These findings suggest that myo-inositol is compartmentalized within hepatocytes, with a bulk metabolically inert pool and a smaller active pool that equilibrates with extracellular myo-inositol via an energy-independent carrier-mediated mechanism, and is preferentially available for efflux or for synthesis of phosphoinositides.
AB - myo-Inositol uptake and conversion to phosphatidylinositol (PI) was studied in isolated rat hepatocytes. Uptake of myo-[2-3H]-inositol into the trichloroacetic acid (TCA)-soluble fraction showed no evidence of saturation, while incorporation into lipid had an apparent Km of 0.28 mmol/L for external myo-inositol. With 50 μmol/L myo-[2-3H]-inositol, approximately half of the radiolabel was found in lipid at 30 minutes. Glucose and galactose were weak inhibitors, while phlorizin at 1 mmol/L reduced uptake by 50%. Metabolic inhibitors reduced incorporation of myo-[2-3H]-inositol into lipid, but had no effect on uptake. Hepatocytes maintained myo-inositol levels of 0.4 mmol/L for 60 minutes when incubated with 50 μmol/L myo-inositol, but levels increased when incubated with 1 mmol/L myo-inositol. Efflux of label was studied in hepatocytes prelabeled for 20 minutes with myo-[2-3H]-inositol. Loss of label was initially rapid, but had slowed by 20 minutes, with much of the label remaining in the cells. Phlorizin inhibited the loss of myo-[2-3H]-inositol, while increasing myo-inositol concentration in the medium enhanced efflux. The effects of these agents on the rate of efflux was found in lipid rather than in the TCA-soluble myo-inositol fraction. These findings suggest that myo-inositol is compartmentalized within hepatocytes, with a bulk metabolically inert pool and a smaller active pool that equilibrates with extracellular myo-inositol via an energy-independent carrier-mediated mechanism, and is preferentially available for efflux or for synthesis of phosphoinositides.
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U2 - 10.1016/0026-0495(93)90093-4
DO - 10.1016/0026-0495(93)90093-4
M3 - Article
C2 - 8387625
AN - SCOPUS:0027400096
SN - 0026-0495
VL - 42
SP - 395
EP - 401
JO - Metabolism
JF - Metabolism
IS - 3
ER -