MacDonald M J, Marshall L K
University of Wisconsin Childrens Diabetes Center, Madison, USA.
Mol Cell Biochem. 2001 Apr;220(1-2):117-25. doi: 10.1023/a:1010821821921.
We studied a mouse doubly homozygous for mutations in the genes encoding malic enzyme (EC 1.1.1.40) and cytosolic glycerol phosphate dehydrogenase (EC 1.1.1.8) (cGPD). This mouse, which we call the mmgg mouse and which is the product of intercrosses between the Mod-1 mouse and the BALB/cHeA mouse, lacks activity of both enzymes. Like both parental strains the mmgg mouse is completely normal in appearance. cGPD is one of the two enzymes that catalyze the reactions of the glycerol phosphate shuttle. The activity of the other enzyme of the glycerol phosphate shuttle, mitochondrial glycerol phosphate dehydrogenase (EC 1.1.99.5) (mGPD), is abundant in tissues, such as brain, skeletal muscle and the pancreatic islet, suggesting that the glycerol phosphate shuttle is important in these tissues which rapidly metabolize glucose. Cytosolic malic enzyme activity is important for shuttles which transport NADPH equivalents from mitochondria to the cytosol. The major finding of the study was a highly abnormal metabolite pattern in tissues of the mmgg mouse suggesting a block in the glycerol phosphate shuttle due to cGPD deficiency. The metabolite pattern did not suggest that malic enzyme deficiency caused an abnormality. Tissue levels of glycerol phosphate (low) and dihydroxyacetone phosphate (high) were only abnormal in skeletal muscle. Glycolytic intermediates, situated at or before the triose phosphates in the pathway, such as fructose bisphosphate and glyceraldehyde phosphate were increased depending on the tissue. Taken together with previous extensive data on the mouse deficient only in cGPD, this suggests a block in glycolysis at the step catalyzed by glyceraldehyde phosphate dehydrogenase caused by an abnormally low NAD/NADH ratio resulting from a nonfunctional glycerol phosphate shuttle. Consistent with this idea the lactate/pyruvate ratio was high in skeletal muscle signifying a low cytosolic NAD/NADH ratio. The mmgg mouse was normal in all other factors studied including blood glucose and serum insulin levels, pancreatic islet mass, insulin release from isolated pancreatic islets, as well as the activities of five metabolic enzymes, including mGPD, in liver, kidney, skeletal muscle and pancreatic islets. cGPD enzyme activity was undetectable in pancreatic islets, 0.5% of normal in liver, and 2.1% of normal in kidney and skeletal muscle. Malic enzyme activity was undetectable in these same tissues.
我们研究了一种小鼠，其编码苹果酸酶（EC 1.1.1.40）和胞质甘油磷酸脱氢酶（EC 1.1.1.8）（cGPD）的基因发生了双纯合突变。这种小鼠，我们称之为mmgg小鼠，是Mod-1小鼠和BALB/cHeA小鼠杂交的产物，两种酶均缺乏活性。与两个亲本品系一样，mmgg小鼠外观完全正常。cGPD是催化甘油磷酸穿梭反应的两种酶之一。甘油磷酸穿梭的另一种酶，线粒体甘油磷酸脱氢酶（EC 1.1.99.5）（mGPD），在脑、骨骼肌和胰岛等组织中活性丰富，这表明甘油磷酸穿梭在这些快速代谢葡萄糖的组织中很重要。胞质苹果酸酶活性对于将NADPH等价物从线粒体转运到胞质的穿梭途径很重要。该研究的主要发现是mmgg小鼠组织中的代谢物模式高度异常，表明由于cGPD缺乏导致甘油磷酸穿梭受阻。代谢物模式并未表明苹果酸酶缺乏会导致异常。甘油磷酸（低）和磷酸二羟丙酮（高）的组织水平仅在骨骼肌中异常。糖酵解中间产物，位于途径中磷酸丙糖之前或之时，如果糖二磷酸和磷酸甘油醛，根据组织不同而增加。结合之前关于仅cGPD缺陷小鼠的大量数据，这表明由于甘油磷酸穿梭功能异常导致NAD/NADH比值异常低，从而在磷酸甘油醛脱氢酶催化的步骤中糖酵解受阻。与此观点一致，骨骼肌中乳酸/丙酮酸比值很高，表明胞质NAD/NADH比值很低。mmgg小鼠在所研究的所有其他因素方面均正常，包括血糖和血清胰岛素水平、胰岛质量、分离胰岛的胰岛素释放，以及肝脏、肾脏、骨骼肌和胰岛中包括mGPD在内的五种代谢酶的活性。在胰岛中未检测到cGPD酶活性，在肝脏中为正常水平的0.5%，在肾脏和骨骼肌中为正常水平的2.1%。在这些相同组织中未检测到苹果酸酶活性。
