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Biomedical subjects

L A Witters

Publications and source records attributed to L A Witters.

10 recordsLinked to original sources
PubMed1979

Glucagon regulation of protein phosphorylation. Identification of acetyl coenzyme A carboxylase as a substrate.

A hormonally induced change in the covalent phosphorylation state of several enzymes is generally regarded as an important mechanism for hormonal modulation of enzyme activity. We have previously demonstrated that epinephrine stimulates the phosphorylation of a peptide of Mr = 220,000 in adipocytes. Incubation of 32P-labeled cytosolic proteins from adipocytes and hepatocytes with antisera raised against homogeneous chicken and rat liver acetyl coenzyme A carboxylase results in the specific and complete precipitation of the same phosphopeptide. No other major phosphopeptide is specifically precipitated. In hepatocytes, glucagon stimulates the incorporation of 32P into this peptide associated with an inhibition of enzyme activity. These data, coupled with previous studies in adipocytes, suggest that cyclic AMP-dependent protein phosphorylation plays a major role in the regulation of acetyl-CoA carboxylase activity and of fatty acid biosynthesis in adipose tissue and liver.

Acetyl-CoA Carboxylase↗
PubMed1979

Regulation of hepatic free fatty acid metabolism by glucagon and insulin.

The roles of glucagon and insulin in the direct short-term regulation of hepatic free fatty acid (FFA) metabolism were studied in hepatocytes isolated from fed, fasted, and streptozotocin-induced diabetic rats. In fed animals, the principal metabolic product of palmitate metabolism was triglyceride, whereas ketones were the major product in fasted and diabetic animals. Glucagon at physiological concentrations increased ketogenesis and decreased triglyceride synthesis from palmitate in hepatocytes from fed rats at FFA concentrations 1.0 mM or less. Insulin had no effect on FFA metabolism when present as the sole hormone, but could antagonize the actions of submaximal concentrations of glucagon. The metabolism of palmitate in fasted or diabetic hepatocytes was unaffected by either hormone. Ketogenesis from octanoate was also unaffected by hormone addition in all cell types. These data are consistent with a locus of hormonal regulation at a step prior to beta-oxidation of fatty acid. Glucagon and insulin may modulate FFA metabolism by both intrahepatic and extrahepatic mechanisms.

Animals↗
PubMed1979

Insulin and the phosphorylation of intracellular proteins.

Protein phosphorylation is a ubiquitous form of posttranslational protein modification in mammalian cells which often serves to regulate protein function. Insulin alters the activity of a number of enzymes known to be regulated via phosphorylation. With the premise that altered protein phosphorylation might be an obligatory intermediate step in insulin action, we have examined the effects of insulin on the phosphorylation of the major phosphopeptides in adipocytes and hepatocytes. Insulin affects overall protein phosphorylation in two ways: 1) Insulin selectively stimulates the phosphorylation of a major peptide in adipose tissue (MW 123,000) and liver (MW 46,000) through a mechanism independent of cAMP and the cAMP-dependent protein kinase. Net dephosphorylation is not observed with insulin as the sole hormone. 2) Insulin antagonizes cAMP-directed protein phosphorylation. The mechanism of insulin-stimulated phosphorylation and the possible role of this phenomenon in overall insulin action is discussed.

Adipose Tissue↗
PubMed1978

Insulin regulation of hepatic glycogen synthase and phosphorylase.

The relative roles of insulin and glucose in the regulation of hepatic glycogen synthase and phosphorylase were studied in hepatocytes from fed rats. Elevation of extra-cellular glucose led to a rapid decrease in phosphorylase a activity followed by a slower increase in glycogen synthase I activity. A reciprocal and coordinate relationship between phosphorylase inactivation and synthase activation in response to glucose was observed; following initial glucose-induced inactivation of phosphorylase, there was a highly significant linear inverse relationship between residual phosphorylase activity and glycogen synthase activation. Insulin led to a further decrease in phosphorylase activity and a 30-50% additional increase in glycogen synthase activity over that caused by glucose. The effects of insulin required the presence of glucose and served to augment acute glucose stimulation of glycogen synthase and inhibition of phosphorylase. Insulin did not perturb the reciprocal and coordinate relationship between phosphorylase inactivation and synthase activation in response to glucose. The results suggest that the ability of insulin to activate hepatic glycogen synthase can be entirely accounted for by its ability to inactivate phosphorylase.

Animals↗
PubMed1977

Insulin antibodies in the pathogenesis of insulin allergy and resistance.

Insulin allergy developed in a patient treated with beef/pork insulin. Desensitization therapy led to cessation of the allergy, but it was associated with the development of diabetic ketoacidosis with apparent insulin resistance which was successfully treated with fish insulin. The patient's initial serum contained a high titer of anti-insulin immunoglobulin E (IgE) antibody directed primarily against beef insulin. Desensitization therapy with pork insulin was associated with the production of anti-insulin immunoglobulin G (IgG) antibody with highest immunologic reactivity to pork insulin. This IgG antibody may have blocked the interaction of insulin with tissue-fixed IgE antibody but, in addition, led to significant increases in total serum insulin-binding capacity and transient insulin resistance. The favorable clinical response to fish insulin was likely due to the negligible immunologic reactivity of this patient's anti-insulin antibodies with fish insulin. This report suggests that desensitization therapy for insulin allergy can lead to insulin resistance of the immune type.

Animals↗
PubMed1976

Therapeutic failure in familial type II hyperlipoproteinemia.

The extended use of diet and cholestyramine therapy in familial type II hyperlipoproteinemia was examined in patients who previously participated in a short-term, double-blind trial. A striking secondary failure in therapeutic response during 4 yr of use of this therapy was noted with plasma cholesterol rising an average of 15%. A 3 mo, out-patient, follow-up study designed to reinforce patient motivation and dietary and drug adherence resulted in a prompt but partial reversal of this therapeutic deterioration in 16 patients. Additional inpatient studies confirmed that patient noncompliance with the dietary regimen was the major factor responsible for the secondary failure. Cholestyramine together with a low cholesterol diet can be an effective agent in familial type II hyperlipoproteinemia, given a comprehensive program of out-patient follow-up with continued emphasis on dietary principles and drug adherence.

Adult↗