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Based on the findings that proinsulin C-peptide binds specifically to cell membranes, we investigated the effects of C-peptide and related molecules on the intracellular Ca2+ concentration ([Ca2+]i) in human renal tubular cells using the indicator fura-2/AM. The results show that human C-peptide and its C-terminal pentapeptide (positions 27-31, EGSLQ), but not the des (27-31) C-peptide or randomly scrambled C-peptide, elicit a transient increase in [Ca2+]i. Rat C-peptide and rat C-terminal pentapeptide also induce a [Ca2+]i response in human tubular cells, while a human pentapeptide analogue with Ala at position 1 gives no [Ca2+]i response, and those with Ala at positions 2-5 induce responses with different amplitudes. These results define a species cross-reactivity for C-peptide and demonstrate the importance of Glu at position 1 of the pentapeptide. Preincubation of cells with pertussis toxin abolishes the effect on [Ca2+]i by both C-peptide and the pentapeptide. These results are compatible with previous data on C-peptide binding to cells and activation of Na-,K+ATPase. Combined, all data show that C-peptide is a bioactive peptide and suggest that it elicits changes in [Ca2+]i via G-protein-coupled pathways, giving downstream enzyme effects.

A glutathione-dependent formaldehyde dehydrogenase (class I11 alcohol dehydrogenase) has been characterized from Arubidopsis thulium. This plant enzyme exhibits kinetic and molecular properties in common with the class I11 forms from mammals, with a K,, for S-hydroxymethylglutathione of 1.4 pM, an anodic electrophoretic mobility (PI: 5.3-5.6) and a cross-reaction with anti-(rat class I11 alcohol dehydrogenase) antibodies. The enzyme structure, deduced from the cDNA sequence, fits into the complex system of alcohol dehydrogenases and shows that all life forms share the class I11 protein type. The corresponding mRNA is 1.4 kb and present in all plant organs; a single copy of the gene is found in the genome. The class I11 structural variability is different from that of the ethanol-active enzyme types in both vertebrates (class I) and plants (class P), although class P conserves more of the class 111 properties than class I does. Also the enzymatic properties differ between the two ethanol-active classes. Activesite variability and exchanges at essential residues (Leu/Gly57, AsplArgll5) may explain the distinct kinetics. These patterns are consistent with two different metabolic roles for the ethanol-active enzymes, a more constant function, reduction of acetaldehyde during hypoxia, for class P, and a more variable function, the detoxication of alcohols and participation in metabolic conversions, for class I. A sequence motif, Pro-Xaa-IleNal-Xaa-Gly-His-Glu-Xaa-Xaa-Gly, common to all medium-chain alcohol dehydrogenases is defined.

Abstract. Using surface plasmon resonance (SPR) and electrospray mass spectro metry (ESI-MS), proinsulin C-peptide was found to influence insulin-insulin interactions. In SPR with chip-bound insulin, C-peptide mixed with analyte insulin increased the binding, while alone C-peptide did not. A control peptide with the same residues in random sequence had little effect. In ESI-MS, C-peptide lowered the presence of insulin hexamer. The data suggest that C-peptide pro motes insulin disaggregation. Insulin/insulin oligomer μM dissociation constants were determined. Compatible with these findings, type 1 diabetic patients receiving insulin and C-peptide developed 66% more stimula tion of glucose metabolism than when given insulin alone. A role of C-peptide in promoting insulin disaggregation may be important physiologically during exo cytosis of pancreatic β-cell secretory granulae and pharmacologically at insulin injection sites. It is compatible with the normal co-release of C-peptide and insulin and may contribute to the bene ficial effect of C-peptide and insulin replacement in type 1 diabetics.Keywords. Surface plasmon resonance, electrospray ionization mass spectrometry, insulin effect, diabetes type 1, proinsulin C-peptide, insulin disaggregation, insulin hexamer decrease.

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Proinsulin C-peptide is known to bind specifically to cell membranes and to exert intracellular effects, but whether it is internalized in target cells is unknown. In this study, using confocal microscopy and immunostained or rhodamine-labeled peptide, we show that C-peptide is internalized and localized to the cytosol of Swiss 3T3 and HEK-293 cells. In addition, transport into nuclei was found using the labeled peptide. The internalization was followed at 37 degrees C for up to 1 h, and was reduced at 4 degrees C and after preincubation with pertussis toxin. Hence, it is concluded to occur via an energy-dependent, pertussis toxin-sensitive mechanism and without detectable degradation within the experimental time course. Surface plasmon resonance measurements demonstrated binding of HEK-293 cell extract components to C-peptide, and subsequent elution of bound material revealed the components to be intracellular proteins. The identification of C-peptide cellular internalization, intracellular binding proteins, absence of rapid subsequent C-peptide degradation and apparent nuclear internalization support a maintained activity similar to that of an intracrine peptide hormone. Hence, the data suggest the possibility of one further C-peptide site of action.

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Proinsulin C-peptide ameliorates renal and autonomic nerve function and increases skeletal muscle blood flow, oxygen uptake and glucose transport in patients with insulin-dependent diabetes mellitus. These effects have in part been ascribed to the stimulatory influence of C-peptide on Na+,K+-ATPase and endothelial nitric oxide synthase. To evaluate the capacity of C-peptide to insert into lipid bilayers and form ion channels, C-peptide secondary structure and membrane interactions were studied with circular dichroism spectroscopy and size exclusion chromatography. C-peptide is shown to lack a stable secondary structure, both when part of proinsulin and when free in aqueous solution, although the N-terminal third of the peptide exhibits an alpha-helical conformation in trifluoroethanol. Moreover, C-peptide remains disordered in the aqueous solvent in the presence of lipid vesicles, regardless of vesicle composition. In conclusion, C-peptide is unlikely to elicit physiological effects through stable conformation-dependent interactions with lipid membranes.