Biodistribution, activation, and retention of proinsulin-transferrin fusion protein in the liver: Mechanism of liver-targeting as an insulin prodrug.

A recombinant proinsulin-transferrin fusion protein (ProINS-Tf) has been previously reported to be a novel long-lasting INS analog, acting specifically on the inhibition of hepatic glucose output. In this study, we investigated the biodistribution, activation and tissue retention of ProINS-Tf to elucidate its liver targeted anti-diabetic mechanism. The biodistribution study revealed that ProINS-Tf exhibited liver specific accumulation after a single intravenous injection, whereas transferrin (Tf) or insulin (INS) showed relatively even distribution among different organs. The conversion of inactive ProINS-Tf into an active immune-reactive INS-Tf form (irINS-Tf) via a Tf receptor (TfR) mediated process only occurred in the liver, but not in other organs. In addition, ProINS-Tf demonstrated a prolonged retention in the liver after an intravenous injection, suggesting the enhanced association of the bifunctional active form, irINS-Tf, within liver cells. Taken together, our results indicate that ProINS-Tf is a highly liver-targeted INS prodrug with a combination of 3 specific actions in liver cells: (1) TfR-mediated binding and uptake of the prodrug on the cell surface, (2) liver-specific, TfR-mediated conversion of the prodrug into its active form, and (3) the bifunctional binding of the active fusion protein to both Tf and INS receptors in the liver to achieve prolonged retention and thus enhanced anti-diabetic activities.

Proinsulin-transferrin fusion protein as a novel long-acting insulin analog for the inhibition of hepatic glucose production.

Proinsulin-transferrin (ProINS-Tf) fusion protein was evaluated for its in vivo pharmacokinetics, efficacy, and mechanism. Our previous studies have shown that ProINS-Tf was converted to active insulin-transferrin (INS-Tf) via the transferrin (Tf)-receptor-mediated pathway in hepatoma cells. We hypothesized that this fusion protein can be administered as a prodrug and be converted to a biologically active protein with specificity for the liver versus other insulin (INS)-sensitive tissues (muscle and adipose). Administration as an inactive prodrug with liver-specific action compared with other INS-sensitive tissues conceivably reduces negative side effects seen with other INS analogs. In this report, the data show that ProINS-Tf exhibited a slow, but sustained, in vivo hypoglycemic efficacy and long plasma half-life. The fusion protein showed activity in the liver, as evidenced by decreased expression of two key hepatic glucose production (HGP) enzymes, PEPCK and glucose-6-phosphatase, and increased glycogen levels under feeding conditions. Furthermore, the INS receptor (IR) phosphorylation (activation) in liver and muscle tissues was compared with postinjection of INS or ProINS-Tf. While INS activated IR in both the liver and muscle, ProINS-Tf only showed activation in the liver. Thus, ProINS-Tf fusion protein can potentially be administered as a prodrug with sustained Tf-mediated activation and selectivity in inhibiting HGP.

Insulin fibrillation and protein design: topological resistance of single-chain analogs to thermal degradation with application to a pump reservoir.

Insulin is susceptible to thermal fibrillation, a misfolding process that leads to nonnative cross-ß assembly analogous to pathological amyloid deposition. Pharmaceutical formulations are ordinarily protected from such degradation by sequestration of the susceptible monomer within native protein assemblies. With respect to the safety and efficacy of insulin pumps, however, this strategy imposes an intrinsic trade-off between pharmacokinetic goals (rapid absorption and clearance) and the requisite physical properties of a formulation (prolonged shelf life and stability within the reservoir). Available rapid-acting formulations are suboptimal in both respects; susceptibility to fibrillation is exacerbated even as absorption is delayed relative to the ideal specifications of a closed-loop system. To circumvent this molecular trade-off, we exploited structural models of insulin fibrils and amyloidogenic intermediates to define an alternative protective mechanism. Single-chain insulin (SCI) analogs were shown to be refractory to thermal fibrillation with maintenance of biological activity for more than 3 months under conditions that promote the rapid fibrillation and inactivation of insulin. The essential idea exploits an intrinsic incompatibility between SCI topology and the geometry of cross-ß assembly. A peptide tether was thus interposed between the A- and B-chains whose length was (a) sufficiently long to provide the "play" needed for induced fit of the hormone on receptor binding and yet (b) sufficiently short to impose a topological barrier to fibrillation. Our findings suggest that ultrastable monomeric SCI analogs may be formulated without protective self-assembly and so permit simultaneous optimization of pharmacokinetics and reservoir life.

Cell itinerary and metabolic fate of proinsulin in rat liver: in vivo and in vitro studies.

Proinsulin, the insulin precursor in pancreatic beta-cells, displays a slower hepatic clearance than insulin and exerts a more prolonged metabolic effect on liver in vivo. To elucidate the mechanisms underlying these differences, the cellular itinerary and processing of proinsulin and insulin in rat liver have been comparatively studied using cell fractionation. As [125I]-insulin, [125I]-proinsulin taken up into liver in vivo was internalized and accumulated in endosomes, in which it underwent dissociation from the insulin receptor and degradation in a pH- and ATP-dependent manner. However, relative to [125I]-insulin, [125I]-proinsulin showed a delayed and prolonged in vivo association with endosomes, a slower in vivo and cell-free endosomal processing, and a higher cell-free endosome-lysosome transfer. Endosomal extracts degraded to a lesser extent proinsulin than insulin at acidic pH; so did, and even proportionally less, at neutral pH, plasma membrane and cytosolic fractions. Proinsulin degradation products generated by soluble endosomal extracts were isolated by HPLC and characterized by mass spectrometry. Under conditions resulting in multiple cleavages in insulin, proinsulin was cleaved at eight bonds in the C peptide but only at the Phe24-Phe25 bond in the insulin moiety. As native insulin, native proinsulin induced a dose- and time-dependent endocytosis and tyrosine phosphorylation of the insulin receptor; but at an inframaximal dose, proinsulin effects on these processes were of longer duration. We conclude that a reduced proteolysis of proinsulin in endosomes, and probably also at the plasma membrane, accounts for its slower hepatic clearance and prolonged effects on insulin receptor endocytosis and tyrosine phosphorylation.

Time-action profiles of the intermediate-acting insulin analogue des(64,65)-human proinsulin.

Des(64,65)-proinsulin (DPRO) is one of several endogenous intermediates arising during the conversion of proinsulin to insulin. In pharmaceutic preparations it is a clear solution containing no other proteins. Animal experiments and preliminary human studies indicated that DPRO should have a protracted time-action profile similar to that of NPH-insulin. Accordingly, we compared the time-action profiles of these two preparations, using the euglycaemic glucose clamp-technique in 9 healthy male volunteers. Different doses of DPRO (0.1, 0.15, 0.2 U/kg) or equipotent doses of NPH ( 0.2, 0.3, 0.4 U/kg) were injected subcutaneously into the abdominal wall. The maximal metabolic effect (GIRmax) of DPRO was greater than that of NPH-insulin (p < 0.05). With increasing doses, GIRmax differed significantly for DPRO but not for NPH-insulin. The time to maximal metabolic effect (tmax) was similar for the three doses of either preparation. However, tmax was reached 30 min earlier with DPRO than with NPH-insulin (p < 0.01). the decline to half-maximal after maximal activity was significantly faster with DPRO than with NPH-insulin (p < 0.0001). Subcutaneous injection of DPRO thus produced a time-action profile between that of regular insulin and NPH-insulin.

Dose-response characteristics of human proinsulin and insulin in non-insulin-dependent diabetic humans.

We compared the actions of human proinsulin and insulin on glucose turnover and on intermediary carbohydrate and lipid metabolism in non-insulin-dependent diabetes mellitus (NIDDM). Six diet-controlled weight-matched (25.4 +/- 1.0 kg/m2) NIDDM subjects underwent six separate isoglycemic clamps. Glucose turnover was measured using a primed continuous infusion of [6',6'-2H2]glucose. Each subject received three low-dose intravenous infusions of both insulin and proinsulin. Blood glucose was maintained at 6.7 +/- 0.3 mM during proinsulin and insulin infusion. Insulin (I) infusions gave steady-state levels of 0.12 +/- 0.001 (I1), 0.18 +/- 0.01 (I2), and 0.33 +/- 0.01 nM (I3). Steady-state proinsulin (P) levels were 2.5 +/- 0.1 (P1), 4.3 +/- 0.2 (P2), and 8.8 +/- 0.9 nM (P3). Hepatic glucose production was suppressed equally by proinsulin and insulin at all doses. The metabolic clearance rate of glucose was significantly increased during the insulin infusion compared with proinsulin. The use of [6',6'-2H2]glucose resulted in a mean underestimation of the glucose infusion rate of 10.0 +/- 4.0 and 6.0 +/- 2.5% during the two highest insulin and proinsulin doses, respectively. Proinsulin had a significantly weaker effect than insulin, at the lowest infusion dose, in percent suppression of plasma nonesterified fatty acids, blood glycerol, and beta-hydroxybutyrate levels (all P less than 0.05). Blood lactate levels were lower during the P1 (628 +/- 43 microM) and P2 (657 +/- 93 microM) infusions compared with I1 (776 +/- 60 microM) and I2 (878 +/- 44 microM; P less than 0.05, P less than 0.02), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Proinsulin as a possible therapeutic agent.

Gene technology has made significant amounts of biosynthetic human proinsulin (BHP) for biological and clinical studies available. It has been shown both in vitro and in vivo that BHP is about 10% as active as biosynthetic human insulin. More specifically it is 8% as potent regarding its action in stimulating peripheral glucose disposal and 12% as potent in suppressing hepatic glucose output. Its hypoglycemic effect is rather produced by the intact molecule or its intermediates and not by conversion to insulin or C-peptide. BHP appears to exert a more prolonged action when administered subcutaneously compared to NPH insulin. Furthermore because of its weaker effect on peripheral glucose utilization it is expected to create less profound hypoglycemia. Under conditions of equipotency BHP may stimulate triglyceride synthesis in a lower degree and in that way be less lipogenic. On longterm basis BHP proved to have a low immunogenic potential. We investigated the efficacy of BHP in split doses in type II diabetics with relative insulin resistance. Glucose levels at 1600 and 2000 and 0400 hrs as well as integrated glycemia over the 24 h period were lower under BHP. The potency of BHP in this and other studies has been demonstrated after application of greater quantities from it in terms of equimolarity. Nevertheless there are certain features such as the protracted action, the stronger effect on hepatic glucose production and the lower suppression of endogenous insulin secretion which make proinsulin a promising agent in the future management of type II diabetes.

Hypoglycemic potency and metabolic clearance rate of intravenously administered human proinsulin and metabolites.

Since circulating proinsulin has been suggested to be important in the pathogenesis of noninsulin-dependent diabetes, and biosynthetic human proinsulin (HPI) may have a therapeutic role in patients with diabetes mellitus, the biological activity of proinsulin metabolites is of potential significance. Moreover, recent studies have suggested that the majority of circulating proinsulin immunoreactivity consists of metabolites. We, therefore, compared the blood glucose-lowering ability and MCR of the two proinsulin metabolites des-(31,32)HPI and des-(64,65)HPI with intact HPI in seven anesthetized dogs after an overnight fast. Intravenous bolus injections of 12.5 micrograms HPI/kg BW and equimolar amounts of des-(31,32)HPI and des-(64,65)HPI were given on three separate occasions. In addition to blood glucose, des-(31,33)HPI, des-(64,65)HPI, and HPI were measured using an insulin RIA and peptide-specific standard curves. Kinetic parameters were derived by fitting two exponentials to the respective decay curves. The MCR of HPI (3.3 +/- 0.1 ml/kg.min) was significantly lower (P less than 0.05) than that of des-(64,65)HPI (6.4 +/- 0.6 ml/kg.min), but was not significantly different from that of des-(31,32)HPI (3.8 +/- 0.4 ml/kg.min). The MCR of biosynthetic insulin (17.2 +/- 1.8 ml/kg.min), as measured in three of the dogs, was higher than that of HPI or the two metabolites. The blood glucose-lowering ability (defined as nadir glucose/fasting glucose, expressed as a percentage) of des-(64,65)HPI (49.3 +/- 5.0%) was significantly greater (P less than 0.05) than that of intact HPI (87 +/- 2.2%), and the glucose-lowering ability of des-(31,32)HPI (75.2 +/- 3.8%) was intermediate. In conclusion, HPI metabolites are more biologically active than intact HPI. The extent of in vivo conversion of proinsulin to metabolites may enhance the biological activity of proinsulin and, thus, have physiological, pathophysiological, and therapeutic significance.

Scintigraphic distribution of 123 I labelled proinsulin, split conversion intermediates and insulin in rats.

Insulin, biosynthetic human proinsulin and 2 human proinsulin conversion intermediates, des (64, 65) human proinsulin and des (31, 32) human proinsulin, were labelled with 123 I and the derivatives monosubstituted on Tyr A14 were purified by reverse phase high performance liquid chromatography. The four tracers were injected into anaesthetized rats via a jugular or a portal vein and time activity curves were generated for the liver and kidneys using a gamma camera and an online computer. Liver extraction coefficients varied in the order insulin (38%), des (64, 65) human proinsulin (11.7%), des (31, 32) human proinsulin (3.2%), human proinsulin (1.6%); whereas half-life of hepatic activity varied in the reverse order, from 6 min for insulin, to 45 min for human proinsulin. As expected for a non-receptor mediated process, kidney extraction varied conversely to liver extraction, being highest for human proinsulin and lowest for insulin. It is concluded that the kinetics of human proinsulin conversion intermediates depends upon the site of cleavage and deletion and is intermediate between those of insulin and intact human proinsulin.

Bioactivity and pharmacokinetics of human proinsulin in comparison to human insulin after intravenous and subcutaneous injection.

The hypoglycemic actions of human insulin (1 IU/kg b.w.) and biosynthetic human proinsulin in about equimolar amounts were studied after intravenous and subcutaneous injection in rabbits. Blood samples were taken up to four hours after injection for the determination of blood glucose and immunoreactive levels of both insulin and human C-peptide. For the determination of human C-peptide, serum taken after proinsulin injection was divided into two fractions. One was examined directly by the human C-peptide radioimmunoassay and the other after incubation with a protein-A-sepharose coupled insulin antibody to find "free human C-peptide". Proinsulin in amounts equimolar to 1 IU insulin/kg b.w., exerted a 34% stronger hypoglycemic action after subcutaneous injection than after intravenous administration (area under curve estimation). Proinsulin-induced hypoglycemia did not last longer after intravenous administration than that induced by intravenous insulin. Although subcutaneous proinsulin did not show the same maximum decrease of blood glucose compared to subcutaneous insulin, its action was significantly prolonged (up to 180 min). Specific measurement of free human C-peptide showed no evidence of conversion of proinsulin to insulin and C-peptide.

Human proinsulin: bioactivity and pharmaco-kinetics after intravenous and subcutaneous administration.

The hypoglycemic actions of biosynthetic human proinsulin and human insulin (1 IU insulin/kg b.w. and about equimolar amounts of proinsulin) were studied after intravenous and subcutaneous injection in rabbits. Blood samples were taken up to four hours after injection for the determination of blood glucose and immunoreactive levels of both insulin and human C-peptide. For the determination of human C-peptide, serum taken after proinsulin injection was divided into two fractions. One was examined directly by the human C-peptide radioimmunoassay and the other after incubation with a protein-A-sepharose coupled insulin antibody to find "free human C-peptide". In amounts equimolar to 1 IU insulin/kg b.w., proinsulin exerted an about one third stronger hypoglycemic action (area under curve estimation) after s.c. compared to i.v. injection. Proinsulin-induced hypoglycemia did not last longer after intravenous administration than that induced by intravenous insulin. Although subcutaneous proinsulin did not show the same maximum decrease of blood glucose as subcutaneous insulin, its action was significantly prolonged (up to 180 min). Specific measurement of free human C-peptide showed no evidence of conversion of proinsulin to insulin and C-peptide.

Investigation of the prolonged hypoglycemic effect of human proinsulin in diabetes mellitus.

Human proinsulin possesses a clear hypoglycemic effect, the extent and kinetics of which were investigated in 8 insulin dependent diabetics. In addition, to adequate therapy with a controlled-release insulin, each patient received i.v. injections of insulin (e U) and proinsulin (9 U) on separate test days. After 9 U of proinsulin, the blood glucose decrease was delayed in comparison to 3 U of insulin. However, it reached identical maximum blood glucose decrease values. The elimination kinetics of proinsulin are delayed compared to insulin. There is a linear relation between the time and the reciprocal value of the measured insulin immuno-reactivity. The hypoglycemic effect of proinsulin which is protracted for hours corresponds to that of an extreme controlled-release insulin. Under certain conditions, proinsulin might therefore prove effective in the long-term treatment of the insulin-dependent diabetes mellitus.

Kinetics of biosynthetic human proinsulin in patients with terminal renal insufficiency.

Eight volunteers with terminal renal insufficiency having consented to the investigation, were given an i.v. bolus administration of 40 pmol biosynthetic human proinsulin on their dialysis-free day. Intravenous blood for the determination of blood glucose proinsulin, insulin and C-peptide was collected in short intervals for 6 hours and thereafter in longer intervals for 24 hours. Proinsulin was determined by immunoradiometric assay with monoclonal antibodies. The proinsulin kinetics were compared with the kinetics of normal volunteers. The behaviour of proinsulin concentration-time is best described with a 3-compartment model. The dominant biological half-life in terminal renal insufficiency was 6.8 hours which signifies a 4.4-fold increase of the normal half-life. The distribution volumes (V1) in the central compartment do not differ in the two groups, whereas the distribution volume after complete distribution (Vss) is significantly increased in renal insufficiency. The total metabolic clearance in renal insufficiency namely 0.63 ml/kg/min is 2.6 times lower compared to normal subjects with 1.67 ml/kg/min. The extra-renal clearance is 39% of the total metabolic clearance rate, whereas the renal clearance comprises 61%. Peripheral conversion from proinsulin to insulin and C-peptide does not occur in terminal renal insufficiency. The basal endogenous proinsulin secretion rate in renal insufficiency does not differ from that of normal volunteers. The following conclusions can be drawn: 1) Hyperinsulinism observed in renal insufficiency can be explained by circulating proinsulin. 2) In the potential therapeutic use of biosynthetic human proinsulin in diabetics with renal insufficiency dosis adjustment according to the remaining renal function would probably be required.

Action profile of biosynthetic human proinsulin trials with the BIOSTATOR in metabolically normal subjects.

Studies to examine the pharmacokinetics and pharmacodynamic properties of biosynthetic human proinsulin were conducted with the aid of the "glucose-controlled insulin infusion system BIOSTATOR". Seven metabolically normal healthy volunteers were given subcutaneous injections of 0.1 mg human proinsulin per kg body weight, and the subsequent behaviour of the serum proinsulin concentration was monitored over a period of 21 hours. The drop in the blood-sugar levels was counteracted by corresponding infusions of glucose by the BIOSTATOR via a special clamp technique. The intensity and frequency of the glucose infusions given by the BIOSTATOR equipment enable us to draw conclusions regarding the hypoglycaemic efficacy of human proinsulin. Two to three hours after the subcutaneous injection, the serum proinsulin concentration had reached its plateau-like maximum value. After approximately 5 hours it had dropped to 2/3 of the maximum, and after another 3 to 4 hours it had fallen further to 1/3 of the maximum. It returned virtually to its initial value after a total of 14 to 16 hours. The dextrose infusion rate calculated by the BIOSTATOR reflects these changes in the form of an "action profile". Proinsulin has a mean transit time (MTT) of 322 minutes. This is longer than the MTT of normal insulin (188 minutes) but markedly shorter than that of NPH insulin (625 minutes). If the hypoglycaemic effects of insulin and human proinsulin are compared on the basis of the areas under their respective action profile curves, the hypoglycaemic effect of 1 mg human proinsulin corresponds to 5.2 IU insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of single and combined infusions of human biosynthetic proinsulin and insulin on glucose metabolism and on plasma hormone concentrations in euglycaemic clamp experiments.

Euglycaemic clamp experiments with single or combined infusions of human biosynthetic proinsulin and insulin were performed in 6 healthy, normal weight subjects in order to assess the possibility of antagonistic, additive, or synergistic effects on whole body glucose metabolism. Insulin (i: 2.02 pmol x kg-1 x min-1) or proinsulin (p: 9.26 pmol x kg-1 x min-1) were infused under euglycaemic clamp conditions over 240 min. After 120 min, the infusion rates were doubled (protocols ii and pp), or proinsulin (protocol ip) or insulin (protocol pi) infusions were added. After 240 min of infusing insulin or proinsulin alone, euglycaemia was maintained for an additional 60 min period without hormone infusions to measure the decay of hormone concentrations and of effects on glucose metabolism. Effects on glucose uptake were measured as the glucose infusion rate necessary to maintain euglycaemia. IR-insulin, IR-proinsulin, IR-C-peptide and IR-glucagon were determined by specific radioimmunoassays. After 240 min, similar steady state glucose infusion rates were reached for all protocols (mean +/- SEM, mg x kg-1 x min-1: ii: 10.6 +/- 1.0; ip: 9.1 +/- 0.4; pi: 10.0 +/- 0.9; pp: 8.4 +/- 0.7). The infusion rate with proinsulin alone (pp), however, was significantly smaller than with insulin alone (ii), indicating a somewhat lower effectiveness of the proinsulin dose employed. With all protocols, nonesterified fatty acid and IR-glucagon concentrations were decreased to a similar extent. Steady state hormone concentrations were reached within 30 min of each infusion period.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic effects of intravenous proinsulin.

Proinsulin induced hypoglycemia was characterized in eight healthy male volunteers. Proinsulin cleared slower from the circulation than insulin. The metabolic effects on plasma glucose, free fatty acids, glycerol, 3-hydroxybutyrate, potassium, and phosphate occurred slower. The anti-lipolytic effect of proinsulin was longer than that of insulin (2P less than 0.001). The hormonal responses of epinephrine, norepinephrine, prolactin, hGH, and cortisol were attenuated following proinsulin. The amount of epinephrine secreted during counterregulation and the amount of lactate produced in response to beta-stimulation were both correlated to the fall of plasma glucose (2P less than 0.005). Stress symptoms were milder after proinsulin (2P less than 0.01). The data obtained are consistent with the hypothesis that the slower kinetics of proinsulin cause slower metabolic effects. The assumption of non-insulin-like effects of a differing pattern of insulin-like effects is not necessitated by the results of this study.

Simultaneous assessment of prohormone transport and processing in four separate islet cell types: a combined autoradiographic and biochemical study.

This study was performed to assess the relationships between prohormone transport and processing in separate cell types in pancreatic islet tissue. Anglerfish islets were subjected to pulse-chase incubation with [3H]tryptophan and/or [35S]cysteine. Tissue and media were removed at specific time points during the incubation and prepared for electron microscopic examination or biochemical analysis. Specific islet cell types were identified ultrastructurally using protein A gold immunocytochemistry. Transport of newly synthesized peptides through specific subcellular compartments was monitored using electron microscopic autoradiography. Prohormone-product ratios were established by gel filtration and high-performance liquid chromatography analyses of tissue extracts. Complete analyses were performed on A-cells (source of proglucagon-II, glucagon-II, and glucagon-like peptide-II), B-cells (proinsulin and insulin), D-cells (prosomatostatin-II and somatostatin-28), and S-cells (prosomatostatin-I and somatostatin-14). Transport of newly synthesized peptides proceeded from rough endoplasmic reticulum (RER) to Golgi complex and then to mature secretory granules in all cell types. The transport rate was most rapid in A- and B-cells, slower in S-cells, and slowest in D-cells. The T1/2 for conversion of prohormone to product(s) was shortest in S-cells (150 min), slightly longer in B-cells (155 min), much longer in D-cells (259 min), and greater than 300 min in A-cells. These results demonstrate that the transport/prohormone conversion relationships are unique in each of the islet cell types monitored.

Pharmacokinetics of biosynthetic human proinsulin following intravenous and subcutaneous administration in metabolically healthy volunteers.

40 pmol of biosynthetic human proinsulin was administered to 8 healthy volunteers by intravenous and by subcutaneous route. Following proinsulin administration, venous blood was collected in regular intervals within which proinsulin was determined by a specific radioimmunometric assay with monoclonal antibodies. The proinsulin concentration was determined simultaneously with the insulin and C-peptide radioimmunoassay. Through this investigation the following kinetic parameters were found: The kinetics of the biosynthetic human proinsulin can be best described by the 3-compartment model. The dominant biological half-life was 92 minutes. In intravenous proinsulin administration a proinsulin mean transit time of 80 minutes was found, whereas in subcutaneous administration a proinsulin retention time of 225 minutes was measured. The mean resorption velocity of the subcutaneously applied proinsulin amounted to 145 minutes. Two lag times for subcutaneous resorption can be described, a short one with 9.4 minutes and a long one with 65 minutes. The initial distribution volume for proinsulin was 3.8 l, whereas the distribution volume after complete distribution was 9.3 l. The mean total metabolic clearance was determined with 120 ml/min. Since no difference for the proinsulin concentration was found using the 3 different determination methods a peripheral proinsulin conversion to insulin and C-peptide is not likely. The basal endogenous secretion rate for proinsulin is 68.7 pmol per hour.