P. Garcia-Webb

Basal C-Peptide in the Discrimination of Type I from Type II Diabetes

Basal serum C-peptide concentrations in diabetic patients showed two groups. Diabetic patients with low C-peptide levels (≤ 0.16 nmol/L) have clinical characteristics of type I diabetes, and all were on insulin therapy. With long duration of diabetes, an increasing proportion had undetectable C-peptide. Diabetic patients with high C-peptide levels (< 0.16 nmol/L) resemble type II diabetes. In this group 30% were on insulin therapy but duration of known disease was not associated with any decline in the high basal C-peptide levels. The small proportion of diabetic patients with basal serum C-peptide in the range of 0.17–0.32 nmol/L have indeterminate status.

Clinical Criteria that Reflect C-Peptide Status in Idiopathic Diabetes

Mako, M. E., Reynolds, C., and Molnar, G. D.: C-peptide suppression test for insulinoma. J. Lab. Clin. Med. 1977; 90:180-86. 5 Service, F. J., Rubenstein, A. H., and Horwitz, D. L: Cpeptide analysis in diagnosis of factitial hypoglycemia in an insulindependent diabetic. Mayo Clin. Proc. 1975; 50:697-701. 4 Service, F. J., and Palumbo, P. J.: Factitial hypoglycemia— three cases diagnosed on the basis of insulin antibodies. Arch. Intern. Med. 1974; 134:336-40. 5 Scarlett, J. A., Mako, M. L , Rubenstein, A. H., Blix, P. M., Goldman, J., Horwitz, D. L , Tager, H., Jaspan, J. B., Stjernholm, M. R., and Olefsky, J. M.: Factitious hypoglycemia—diagnosis by measurement of serum C-peptide immunoreactivity and insulinbinding antibodies. N. Engl. J. Med. 1977; 297:1029-32. 6 Walfish, P. G., Kashyap, R. P., and Greenstein, S.: Sulfonylurea-induced factitious hypoglycemia in a non diabetic nurse. Can. Med. Assoc. J. 1975; 112:71-72. 7 Jordan, R. M., Kammer, H., and Riddle, M. R.: Sulfor.ylureainduced factitious hypoglycemia: a growing problem. Arch. Intern. Med. 1977; 137:390-93. 8 Ahlquist, D. A., Nelson, R. L., and Callaway, C. W.: Pseudoinsulinoma syndrome from inadvertent tolazamide ingestion. Ann. Intern. Med. 1980; 93:281-82.

Inhibition of human lymphocyte ferrochelatase activity by hemin

Ferrochelatase activity in human lymphocytes was found to be 50% inhibited by 10.5 microM hemin under maximal velocity conditions. The inhibition was not prevented by dithiothreitol or glutathione, suggesting that the hemin was not interacting with the sulphydryl groups of ferrochelatase. Human serum albumin, but not bovine serum albumin was able to prevent the inhibition consistent with the known formation of the tightly bound methemalbumin complex with human albumin. Kinetic studies performed under initial velocity conditions with hemin concentrations ranging from 2 to 8 microM revealed the inhibition to be non-competitive with respect to the metal substrate (zinc) and competitive with respect to the porphyrin substrate (mesoporphyrin). The kinetic analysis indicated that hemin binds to both the enzyme and enzyme-metal complex at a site normally occupied by the porphyrin substrate, and a second molecule of hemin could bind to the enzyme-metal complex but with a much lower affinity than the first molecule. We conclude that the product inhibition of ferrochelatase by hemin should be considered as a possible site of regulation of heme biosynthesis.

The Effects of Dietary Calcium Deprivation on Serum Calcitriol Levels in Premenopausal and Postmenopausal Women

The effect of the postmenopausal state on serum calcitriol levels was studied in 13 normal premenopausal women and 10 normal postmenopausal women under basal conditions and during seven days of dietary calcium deprivation. Calcitriol levels and the free calcitriol index were lower in the postmenopausal women, and this difference persisted during calcium deprivation. One of the major actions of calcitriol is to increase gut calcium absorption. Relative calcitriol deficiency, despite retained renal synthetic reserve, would in part explain the increased dietary calcium requirement to maintain calcium balance in postmenopausal women.

Correlation between fasting serum C-peptide and B cell insulin secretory capacity in diabetes mellitus

Sir, 3.0 In subjects with Type 1 (insulin-dependent) diabetes an indication of B cell insulin secretory capacity may be obtained by measuring serum C-peptide reactivity before and after an appropriate stimulus. Of the agents used, glucagon has been stated to be the most suitable [1]. A significant correlation between fasting and stimulated serum C-peptide reactivity has been observed [1, 2] and denied 2 .5 [3]. To clarify these differences, we measured B cell secretory capacity in 16 subjects with Type 1 diabetes whose age at onset of disease was less than 40 years (group 1), 18 subjects with Type 1 diabetes whose age at onset of disease was 40 years or more (group 2) and 22 subjects with Type 2 (non-insulin dependent) diabetes whose age at onset of disease was 40 years or more (group 3). Serum C-peptide reactivity was measured [4] in the fasting state, and 3, 6 and 10 rain 2 .0 after the end of a 3-rain IV infusion of 1 mg of glucagon. Only suber jects showing a significant post-glucagon increase in serum C-pepft.

Insulin binding and degradation in isolated hepatocytes from streptozotocin injected rats

Isolated hepatocytes from streptozotocin injected rats bound the same amount of [125I]monoiodoinsulin as hepatocytes from control rats. Scatchard analysis confirmed that insulin receptor number and affinity were the same for both groups. Relatively more cell-associated radioactivity was located intracellularly in hepatocytes from streptozotocin injected rats. Pretreatment with chloroquine resulted in a smaller increase in intracellular [125I]monoiodoinsulin in cells isolated from streptozotocin injected rats than for control cells. These results suggest that intracellular insulin processing occurs more slowly in hepatocytes isolated from streptozotocin injected rats than from control rats.

Inhibition of human lymphocyte coproporphyrinogen oxidase activity by metals, bilirubin and haemin

Coproporphyrinogen III oxidase activity in human lymphocytes was found to be inhibited by cadmium and mercury but not by lead. The organo-metal compounds tributyltin and methylmercury were effective inhibitors of this haem biosynthetic pathway enzyme. Haemin (the ultimate product of the pathway) and bilirubin (a product of haem catabolism) were also shown to be inhibitory. Kinetic studies performed under initial velocity conditions showed that bilirubin was a non-competitive inhibitor and that one bilirubin molecule was bound to both the enzyme and enzyme substrate complex. The analysis also showed haemin to be a non-competitive inhibitor in which two haemin molecules bind to the enzyme whereas the enzyme substrate complex accepts only one haemin molecule. The possible physiological significance of the inhibition of coproporphyrinogen III oxidase activity by haemin and bilirubin is discussed.

Metal inhibition of ferrochelatase activity in human lymphocytes

The activity of the terminal enzyme of haem biosynthesis, ferrochelatase (EC 4.99.1.1) was measured in sonicates of human lymphocytes. We used a sensitive method with zinc and mesoporphyrin as substrates and quantification of the product zinc-mesoporphyrin by HPLC. A variety of metal ions and organometal compounds were examined as possible inhibitors of ferrochelatase activity. Inhibition was observed with copper and mercury (but not with lead) and with tributyltin and methylmercury. The kinetics of ferrochelatase inhibition were examined for each of the four inhibitors identified.

Factors affecting fasting serum C-peptide levels in micronesians: comparison with a caucasoid population

SummaryFasting serum C-peptide immunoreactivity was determined on Nauruans, a Micronesian population with a high prevalence of diabetes. In Micronesian subjects neither age nor gender had a significant effect on fasting serum C-peptide. In non-diabetic subjects, as has been shown previously for Caucasiod subjects, both obesity and fasting plasma glucose levels were determinants of fasting serum C-peptide. Obesity was the major determinant. Taken overall, mean fasting serum C-peptide increased then possibly fell in subjects grouped by increasing 2-h post-glucose plasma glucose levels. Mean fasting serum C-peptide in newly-diagnosed diabetic subjects was greater than that in non-diabetic subjects with a similar degree of obesity, supporting the concept that the transition to diabetes may be associated with an increase in insulin resistance. The data for non-diabetic subjects were compared with serum C-peptide measured in the same laboratory on samples from a Caucasoid population in Busselton, Western Australia. There was no difference in fasting serum C-peptide level between Micronesian and Caucasoid subjects approximately matched for obesity and fasting plasma glucose levels.

Studies on the inhibitory effect of bacitracin on 125I-labelled insulin internalization in the rat hepatocyte

Previous studies have suggested that transglutaminase has a role in the internalization of some polypeptide hormones and is inhibited by the antibiotic, bacitracin. Bacitracin has been used in insulin-receptor studies to inhibit extracellular degradation of 125I-labelled insulin. The aim of this study was to investigate bacitracins effect on 125I-labelled insulin-receptor interactions in isolated rat hepatocytes. 1 g/l bacitracin increased cell-associated 125I-labelled insulin insulin at 20, 30 and 37 degrees C (P less than 0.001, 0.0005 and 0.0005, respectively). At 5 and 15 degrees C (internalization does not occur), bacitracin did not affect cell-associated 125I-labelled insulin. The bacitracin effect was concentration dependent, increasing to 2 g/l. Scatchard analysis showed that bacitracin did not alter insulin receptor affinity or number. 1 g/l bacitracin abolished the effect of chloroquine. The increased cell-associated radioactivity with bacitracin was surface-bound in nature. 0.5 g/l bacitracin decreased 125I-labelled insulin degradation in hepatocyte suspensions (P less than 0.001) and in buffer previously incubated with hepatocytes (P less than 0.0005). More 125I-labelled insulin remained associated with cells during dissociation studies at 37 degrees C when the buffer contained 1 g/l bacitracin. Label that appeared in the buffer after 60 min was significantly more intact in the presence of bacitracin (P less than 0.025). These results suggest that bacitracin retards the internalization of 125I-labelled insulin in isolated rat hepatocytes.