Investigating the Binding of Albumin and C‐peptide to Red Blood Cells

Abstract

The pancreatic β‐cells of healthy individuals secrete insulin and C‐peptide, a 31‐amino acid peptide, in a 1:1 mole ratio. However, the β‐cells of people with type 1 diabetes (T1D) are damaged or destroyed. Therefore, exogenous insulin is required for people with T1D to survive. Even with regular insulin treatments, patients develop complications such as retinopathy, neuropathy, and nephropathy. Research from our group demonstrates that C‐peptide increases microvascular blood flow, therefore, it could be a useful tool to decrease these complications due to poor blood flow. However, the unknown C‐peptide receptor and mechanism has been a major roadblock in utilizing C‐peptide as a therapeutic. Utilizing ELISA, our group has demonstrated that approximately 1,800 C‐peptide molecules bind per red blood cell (RBC) in the presence of albumin. Without albumin, there was no detectable C‐peptide binding per RBC. Rather than C‐peptide solely binding to a RBC receptor, we hypothesize that C‐peptide binds to RBCs through an albumin/C‐peptide complex receptor. Bovine serum albumin (BSA) binding per RBC was detectable through an attached gamma decay radiolabel (99mTc). A binding saturation experiment was conducted to examine the specific binding of BSA to RBCs with and without C‐peptide. The specific binding curves revealed that albumin saturates at 14,021 ± 1489 BSA molecules/RBC with a Kd of 1.07 (±0.19) × 10−7 M and a Bmax of 1.94 (±0.06) × 10−8 M, or 13,900 receptor molecules/RBC. Whereas, in the presence of C‐peptide and zinc, albumin saturates at 16,696 ± 1479 BSA molecules/RBC with a Kd of 1.91 (±0.09) × 10−7 M and a Bmax of 2.50 (±0.03) × 10−8 M, or 17,900 receptor molecules/RBC. At saturation, the additional 2,700 BSA molecules binding per RBC in the presence of C‐peptide and zinc indicates that an albumin receptor exists on RBCs as well as a separate albumin/C‐peptide complex receptor.Support or Funding InformationNIH