Changes of serum proteins in rats with streptozotocin (STZ) -induced diabetes were analyzed by two-dimensional electrophoresis in non-denaturing conditions ; marked decreases or increases of several specific proteins were detected. W e report here, characterizations of four of those proteins (P
1to P
4), that changed especially after STZ administration. P
1, with pI 4.4, decreased, and P
2-P
4, with pI 4.7, 4.2 and 4.7, respectively, increased. All were glycoproteins. Both P
2and P
3were determined by sequential analysis of peptide fragments digested wii: h lysylendopeptidase to be T-kininogen. No sequences coinciding with P
1or P
4were found in the Protein Database. P
4was believed to be an α
2HS-glycoprotein from its two-dimensional electrophoretic position, but had no properties known to human α
2HS-glycoprotein. Purified P
1and rat serum showed a single precipitin line in the α
1-region in immunoelectrophoresis. This suggests that purified P
1is immunochemically homogeneous, and the only protein that reacts with anti-P
1in rat serum is P
1. Anti-P
1formed a precipitin line against mouse serum and partially fused with rat P
1, but did not react with human, dog, cattle or horse sera. Serum P
1concentration in control rats was about 6 mg/ml, and decreased below 50 % within 3 weeks after STZ administration. The P
1concentration recovered when blood glucose was controlled at its normal level with insulin, 3 weeks after STZ administration, and decreased again to original level when rats were again maintained without treatment. The changed P
2-P
4also recovered when blood glucose was controlled at its normal level, suggesting close relation with diabetes, or at least with blood glucose level, after STZ administration.
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