Sun Tie-hua, Heimark Douglas B, Nguygen Thang, Nadler Jerry L, Larner Joseph
Insmed Incorporated, 4851 Lake Brooke Dr., Glen Allen, VA 23060, USA.
Biochem Biophys Res Commun. 2002 May 10;293(3):1092-8. doi: 10.1016/S0006-291X(02)00313-3.
Previous data from our and other labs demonstrated a decreased chiro-inositol content in urine and tissues of human subjects and animals with type 2 diabetes. In urine this decrease in chiro-inositol was accompanied by an increase in myo-inositol content. Decreased urine levels of chiro-inositol in monkeys were next correlated with the severity of underlying insulin resistance determined by five separate assays. To investigate the decreased chiro-inositol and the accompanying increased myo-inositol excretions in urine in humans and monkeys, we postulated a defect in the epimerization of myo-inositol to chiro-inositol. [(3)H]Myo-inositol was then shown to be converted to [(3)H]chiro-inositol in rats in vivo and in fibroblasts in vitro in a process stimulated by insulin. We next demonstrated that the conversion of [(3)H]myo-inositol to [(3)H]chiro-inositol in vivo was markedly decreased in GK type 2 diabetic rats compared to Wistar controls in liver, muscle, and fat, insulin sensitive tissues. Decreases of 20-25% conversion to baseline levels of under 5% conversion were observed. In the present work, we initially compared the total contents of myo-inositol and chiro-inositol in GK type 2 diabetic rat kidney, liver, and muscle compared to Wistar controls. We demonstrated a consistent decreased total chiro-inositol to myo-inositol ratio in kidney, liver, and muscle compared to controls. We next established the presence of a myo-inositol to chiro-inositol epimerase activity in rat liver cytosol. Enzyme activity was shown to be time and enzyme concentration dependent with a broad pH optimum. It required NADH and NADPH for full activity, which is compatible with its action via an oxido-reductive mechanism. Lastly, we demonstrated that the epimerase enzyme bioactivity was significantly decreased in muscle, liver, and fat cytosolic extracts of GK type 2 diabetic rats versus Wistar controls. Decreased myo-inositol to chiro-inositol epimerase activity may therefore play a role in explaining the decreased chiro-inositol to myo-inositol urine and tissue ratios observed here and in previous animal and human studies. Further it may also possibly play a role in the underlying insulin resistance.
我们实验室和其他实验室之前的数据表明，2型糖尿病患者及动物的尿液和组织中手性肌醇含量降低。在尿液中，手性肌醇的这种降低伴随着肌醇含量的增加。接下来，猴子尿液中手性肌醇水平的降低与通过五种不同检测方法确定的潜在胰岛素抵抗严重程度相关。为了研究人类和猴子尿液中手性肌醇减少以及随之而来的肌醇排泄增加的情况，我们推测肌醇向手性肌醇的差向异构化存在缺陷。然后发现，在体内的大鼠以及体外的成纤维细胞中，[（3）H]肌醇在胰岛素刺激的过程中会转化为[（3）H]手性肌醇。接下来我们证明，与Wistar对照相比，GK 2型糖尿病大鼠肝脏、肌肉和脂肪（胰岛素敏感组织）中，体内[（3）H]肌醇向[（3）H]手性肌醇的转化明显减少。观察到转化率从20 - 25%降至基线水平以下的5%。在本研究中，我们首先比较了GK 2型糖尿病大鼠肾脏、肝脏和肌肉与Wistar对照中肌醇和手性肌醇的总含量。我们证明，与对照组相比，肾脏、肝脏和肌肉中手性肌醇与肌醇的总比例持续降低。接下来我们确定了大鼠肝脏胞质溶胶中存在肌醇向手性肌醇的差向异构酶活性。酶活性显示出时间和酶浓度依赖性，最适pH范围较宽。它需要NADH和NADPH才能发挥完全活性，这与其通过氧化还原机制的作用相符。最后，我们证明，与Wistar对照相比，GK 2型糖尿病大鼠肌肉、肝脏和脂肪胞质溶胶提取物中的差向异构酶生物活性显著降低。因此，肌醇向手性肌醇差向异构酶活性降低可能在解释此处以及之前动物和人类研究中观察到的手性肌醇与肌醇尿液及组织比例降低方面发挥作用。此外，它也可能在潜在的胰岛素抵抗中发挥作用。
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