Sigal S H, Yandrasitz J R, Berry G T
Division of Biochemical Development and Molecular Diseases, Children's Hospital of Philadelphia, PA 19104.
Metabolism. 1993 Mar;42(3):395-401. doi: 10.1016/0026-0495(93)90093-4.
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 mumol/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 mumol/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.
在分离的大鼠肝细胞中研究了肌醇的摄取及其向磷脂酰肌醇（PI）的转化。肌醇-[2-³H]摄取到三氯乙酸（TCA）可溶部分中未显示出饱和迹象，而掺入脂质中时，外部肌醇的表观Km为0.28 mmol/L。使用50 μmol/L肌醇-[2-³H]时，在30分钟时约一半的放射性标记物存在于脂质中。葡萄糖和半乳糖是弱抑制剂，而1 mmol/L的根皮苷使摄取减少50%。代谢抑制剂减少了肌醇-[2-³H]掺入脂质，但对摄取没有影响。当与50 μmol/L肌醇一起孵育时，肝细胞在60分钟内维持0.4 mmol/L的肌醇水平，但与1 mmol/L肌醇一起孵育时水平会升高。在用肌醇-[2-³H]预标记20分钟的肝细胞中研究了标记物的流出。标记物的损失最初很快，但到20分钟时减缓，大部分标记物仍留在细胞中。根皮苷抑制肌醇-[2-³H]的损失，而增加培养基中的肌醇浓度则增强流出。发现这些试剂对流出速率的影响存在于脂质部分而非TCA可溶的肌醇部分。这些发现表明，肌醇在肝细胞内被分隔开来，有一个大量代谢惰性池和一个较小的活性池，该活性池通过能量非依赖的载体介导机制与细胞外肌醇平衡，并且优先用于流出或磷酸肌醇的合成。
