Greive Kerryn A, Osicka Tanya M, Russo Leileata M, Comper Wayne D
Department of Biochemistry and Molecular Biology, Monash University, Clayton, Vic, Australia.
Am J Nephrol. 2003 Jan-Feb;23(1):38-46. doi: 10.1159/000066297.
AIMS/HYPOTHESIS: Streptozotocin (STZ) diabetic rats are characterized by the development of albuminuria. It is not known, however, whether the excess excretion of protein is primarily due to intact protein or protein fragments or whether it is specific for albumin or occurs for all high-molecular-weight plasma proteins. To test this we have measured the excretion rates and fractional clearances of [(14)C]albumin, [(3)H]immunoglobulin G and [(3)H]transferrin in diabetic rats.
The radiolabeled proteins were delivered to the circulation of conscious diabetic (STZ induced for 6 weeks) and control rats by ALZET osmotic pumps. The plasma level of the radiolabeled proteins reached steady-state levels by day 7. Urine and plasma samples from day 7 were used to determine the excretion rates of the proteins by radioactivity and radioimmunoassay.
When excretion rates were determined by radioactivity it was apparent that only the albumin excretion rate increased significantly with STZ diabetes to a value of 354 +/- 166 microg/min which agrees with proteinuria determined by Biuret assay of 299.9 +/- 52.4 microg/min. The major proportion of protein being excreted was in the form of protein fragments which are not detected by conventional immmunochemical assays.
The previously unrecognized nephrotic-like levels of proteinuria in experimental diabetes appears to be associated with an albumin-specific mechanism responsible for the increase in albumin peptides in urine. There was significant lowering of plasma albumin concentration but plasma concentrations of transferrin and immunoglobulin G remained unchanged. There was also no significant appearance of intact protein in urine that is normally found in nephrotic states.
目的/假设：链脲佐菌素（STZ）诱导的糖尿病大鼠的特征是出现蛋白尿。然而，尚不清楚蛋白质的过量排泄主要是由于完整蛋白质还是蛋白质片段，也不清楚这是白蛋白特有的现象还是所有高分子量血浆蛋白都会出现。为了验证这一点，我们测量了糖尿病大鼠中[¹⁴C]白蛋白、[³H]免疫球蛋白G和[³H]转铁蛋白的排泄率和分数清除率。
通过ALZET渗透泵将放射性标记的蛋白质输送到清醒的糖尿病（STZ诱导6周）大鼠和对照大鼠的循环系统中。到第7天时，放射性标记蛋白质的血浆水平达到稳态水平。使用第7天的尿液和血浆样本，通过放射性和放射免疫测定法确定蛋白质的排泄率。
通过放射性测定排泄率时，很明显只有白蛋白排泄率在STZ诱导的糖尿病中显著增加，达到354±166微克/分钟，这与通过双缩脲法测定的蛋白尿（299.9±52.4微克/分钟）相符。排泄的蛋白质主要以蛋白质片段的形式存在，而传统免疫化学测定法无法检测到这些片段。
实验性糖尿病中先前未被认识到的类似肾病水平的蛋白尿似乎与一种白蛋白特异性机制有关，该机制导致尿液中白蛋白肽增加。血浆白蛋白浓度显著降低，但转铁蛋白和免疫球蛋白G的血浆浓度保持不变。尿液中也没有出现肾病状态下通常会出现的完整蛋白质。
