Reduced Level of Opioid Peptides, Hemorphin-7 Peptides, in Serum of Diabetic Patients

Hemorphins are endogenous peptides belonging to the family of atypical opioid peptides (1) that are released from sequentially hydrolyzed hemoglobin, the first sequence implicating a hemoglobin cathepsin D proteolysis (2). They were isolated as naturally occurring peptides in various tissues and biological fluids and many of their biological effects have been described (1). Until now, no study had been performed concerning the consequence of the hemoglobin glycosylation on the hemorphin generation in diabetes.

In the present study, the ability of cathepsin D to liberate hemorphin-7 peptides from glycated hemoglobin was performed. To accomplish this, bovine hemoglobin was glycated in vitro and then hydrolyzed by cathepsin D. The hemorphins released (LVV-Hemorphin-7 and VV-Hemorphin-7) were quantified by high-pressure liquid chromatography and compared with the hemorphin level liberated from nonglycated hemoglobin. Moreover hemorphin-7 peptides serum levels between diabetic and nondiabetic patients were compared. Serums from 31 diabetic (aged 47 ± 17 years with a mean HbA_1c 8.4 ± 1.7%) and 25 nondiabetic (aged 39 ± 15 years) patients were estimated by an enzyme-linked immunosorbent assay procedure (3).

Results demonstrated that liberation of LVV-Hemorphin-7 and VV-Hemorphin-7 from in vitro glycated hemoglobin decreased three and five times, respectively, in comparison with normal hemoglobin. Moreover, compared with the control subjects, diabetic patients exhibited significantly lower levels of serum hemorphin-7 peptides (0.8 ± 0.94 vs. 4.09 ± 1.05 μmol/l, P < 0.0001). Nevertheless, no correlation was found between HbA_1c and hemorphin levels.

Consequently, in vivo release of hemorphins from hemoglobin hydrolysis is probably altered by glycosylation, as the present results indicate that the hemoglobin glycosylation reduces its degradation by cathepsin D.

With regards to the many effects attributed to hemorphins in the organism (among which are antihypertensive [4] and opioid-like effects [5]), the results from this study cause one to wonder about the consequence of their reduced level in diabetic patients. Does the diminution of hemorphins released from diabetic serum contribute to the decreased pain threshold to exogeneous or endogeneous nociceptive stimuli? Because the cause of pain in diabetic neuropathy remains uncertain and because its control is the most difficult management issue (6), further studies are required to explore the relation between reduced hemorphin levels and the hyperalgesic forms of peripheral neuropathy.