Diabetes Mellitus Case (Exam II) Flashcards
How does glucose enter most cells and how is this process regulated? How does glucose transport differ in cells that depend on glucose as opposed to those that do not? Which tissues are these? What are the characteristics of glucose transport in liver, a glucose-producing organ?
Glucose passively enters most cells by facilitated diffusion via specific glucose transporters
(GLUTs) which function as uniporters. The transporters differ from tissue to tissue. Tissues which
are dependent on glucose have a transporter which is saturated at physiological glucose concentrations in the blood. Such tissues include erythrocytes, lens of the eye, and brain. Tissues which can use fatty acids (and ketone bodies), such as heart, skeletal muscle, and adipose tissue, are independent of glucose and their glucose transporter (GLUT4) is dependent on insulin for
movement to the plasma membrane. Liver, which produces glucose and uses fatty acids (but not
ketone bodies), has an isoform of the glucose transporter (GLUT2) which is insulin independent, but has a high Km (low affinity) for glucose.
How would you expect hypoinsulinemia to affect blood glucose levels?
In the absence of insulin, uptake by the tissues with insulin-dependent transporters would be limited, thus slowing the utilization of blood glucose and resulting in hyperglycemia.
What are the pathways involved in the liver’s role of regulating blood glucose levels?
An important function of liver is to maintain blood glucose levels within the physiologic range.
When blood glucose levels rise in the absorptive state (up to about 4 hours after a meal), the liver takes up and phosphorylates glucose. Liver uses the resulting glucose 6-phosphate for glycogenesis and lipogenesis. These pathways of storage are active both because the covalent effect (deP) of insulin increases the activity of key enzymes involved, and because the transcriptional effect of insulin increases the amount of key enzymes involved.
[The use of glucose for lipogenesis requires NADPH; thus, the pentose phosphate pathway also is upregulated. Because lipogenesis requires acetyl CoA, glycolysis and the pyruvate dehydrogenase (PDH) reaction are also upregulated.]
When blood glucose (and insulin) levels fall in the fasted state, the liver produces free glucose from glycogenolysis and gluconeogenesis; liver has glucose 6-phosphatase.
These pathways are active because of the covalent (P) and transcriptional effects of glucagon, resulting in increased activity and amount of key enzymes involved.
Liver also provides ketone bodies: ↓ fatty acid synthesis (acetyl CoA carboxylase [ACC] is P by AMPK and inactive), ↓ malonyl CoA so ↑ β-oxidation with ↑ in acetyl CoA and ↑ in ketogenesis.
What hormones regulate glucose metabolism in liver, and how do they act on these pathways?
Insulin and glucagon. Insulin, although not required for the uptake of glucose by liver, does stimulate the storage of glucose as glycogen, and the catabolism of glucose to provide the acetyl CoA needed for fat synthesis. The stimulation by insulin is a consequence of both covalent and transcriptional effects. A high insulin to glucagon ratio suppresses glycogenolysis and gluconeogenesis, while a low ratio activates these processes.
Glucagon causes activation of adenylyl cyclase which activates protein kinase A (PKA). This results in phosphorylation (and activation) of the enzymes of glycogenolysis, and phosphorylation (and inactivation) of glycogen synthase, and of pyruvate kinase and phosphofructokinase-2 (PFK2) in liver.
[Note: Phosphorylation of the kinase domain of hepatic PFK2 results in activation of the phosphatase
domain of the bi-functional enzyme.] Glucagon also results in increased synthesis of phosphoenolpyruvate carboxykinase (PEPCK). So, as with insulin, glucagon has both covalent and transcriptional effects.
What are some of the effects of hyperglycemia?
Glucose, like all small molecules, passes through the glomerular filter in the kidney. Normally all filtered glucose is actively reabsorbed by sodium-linked glucose transporters (SGLT-1 and SGLT-2) of the renal tubule cells. When the blood glucose concentration becomes excessive (> 180 mg/dL), the transporters can no longer recover all the glucose and glucose “spills” into the urine. The very high concentration of glucose in the urine draws water osmotically from the body resulting in polyuria and the loss of both glucose and water.
This may lead to a profound dehydration that can be fatal. (Recall that enterocytes also use a SGLT for the uptake of glucose.)
In addition, at high concentrations glucose may react nonenzymatically (and reversibly) with free amino groups, such as those at the N-terminal amino acid of many proteins. Hemoglobin A1c is an example of this. In the long term, glycation may impair the function of proteins. For example, glycated myelin may contribute to impaired nerve conduction.
Reduction of glucose to sorbitol (a polyol) and its accumulation in tissues with insulinindependent
glucose uptake (e.g., lens of the eye, peripheral nerve, kidney) is thought to play a role in important long-term microvascular complications of diabetes (retinopathy including cataracts, neuropathy, nephropathy). [Recall cataract formation is seen in pathologies of
galactose metabolism that result in increased galactitol, a polyol.]
Hyperglycemia causes increased flux through this “polyol pathway”, resulting in increased amounts of sorbitol and fructose, and decreased NADPH/NADP+ and NAD+/NADH ratios in the cytosol. Increased sorbitol can result in osmotic stress since it doesn’t easily cross membranes. It also can be oxidized to fructose leading to advanced glycation end products (AGE) that irreversibly damage proteins e.g., cross-link them. Decreased NADPH/NADP+ ratio results in decreased ability to maintain glutathione in its reduced (GSH), functional form leading to increased oxidative stress.
Which hormones are responsible for the mobilization of fatty acids from adipose tissue and by what
mechanism?
Major hormonal signals for fatty acid mobilization from adipose tissue include norepinephrine, released by the sympathetic nervous system, and epinephrine, a circulating hormone. (Glucagon also plays a role.) Each acts through cAMP, leading to phosphorylation by PKA and activation of hormone-sensitive lipase (HSL) in adipose. Note, then, that phosphorylation activates lipolysis (PKA ⊕ HSL) and inhibits lipogenesis (AMPK Ɵ ACC).
What is the effect of fatty acid oxidation on gluconeogenesis, the pyruvate dehydrogenase complex,
and the TCA cycle?
The major products of β-oxidation are NADH and acetyl CoA; ultimately, ATP is produced. Acetyl CoA stimulates pyruvate carboxylase, a gluconeogenic enzyme. NADH and ATP are required for gluconeogenesis. Fatty acid oxidation decreases carbohydrate oxidation: NADH and
acetyl CoA inhibit PDHC through stimulation of PDH kinase. NADH inhibits aerobic metabolism by the TCA cycle through its inhibitory effects on isocitrate dehydrogenase and by a decrease in oxaloacetate availability by shifting it to malate.
What influences the rate of fatty acid oxidation and how is the rate affected under type 1 diabetic
conditions?
The rate of fatty acid oxidation is determined at three levels. First the availability of the free fatty acids is critical.
Second, in the liver, the absence of malonyl CoA allows the ready transfer of long chain fatty acids from cytosolic fatty acyl CoA into the mitochondria. Third, the acetyl CoA/CoA ratio or the availability of free CoA controls β-oxidation in muscle, for instance.
In the liver, ketone body formation serves to regenerate free CoA. In type 1 DM, the lack of insulin results in a
rise in glucagon, with a rise in lipolysis and a decrease in lipogenesis; therefore, fatty acids are available for β-oxidation. Oxidation occurs because the inhibitor of fatty acid oxidation, malonyl CoA, is not being made because ACC is phosphorylated (and inactivated) by AMPK
What is the source of ketone bodies? How (and where) are they synthesized? What pushes acetyl CoA to ketogenesis rather than to the TCA cycle in liver?
The source of the ketone bodies is the acetyl CoA produced by oxidation of fatty acids provided to liver by lipolysis in adopose. They are synthesized in the mitochondrial matrix of hepatocytes. The pathway of synthesis involves the condensation of three acetyl CoA molecules to form HMG CoA, which is cleaved by HMG CoA lyase to form acetoacetate and acetyl CoA. The acetoacetate can be non-enzymatically decarboxylated to yield acetone (deadend) or reduced to β-hydroxybutyrate (the 2nd physiologically useful ketone body). The release of CoA means that ketone bodies can cross the inner mitochondrial membrane. The CoA made available supports continued β-oxidation in liver. Note: The rise in NADH from fatty acid oxidation pushes oxaloacetate to malate. The decrease in oxaloacetate pushes acetyl CoA to ketogenesis.
How does the production of ketone bodies affect the ATP yield from fatty acid oxidation in the liver? How might this be compensated for?
The use of acetyl CoA in the formation of ketone bodies and the reoxidation of NADH by formation of β-hydroxybutyrate means that this acetyl CoA and NADH are not available for ATP synthesis. For example, the oxidation of palmitate in the muscle should generate 129 ATP/palmitate. Oxidation of palmitate in liver to four molecules of β-hydroxybutyrate would generate only 21 ATP/palmitate. This decrease in efficiency of ATP production must be compensated by a more rapid rate of β-oxidation.
What is the effect of ketosis on the plasma pH and how does the body compensate? Consider both respiratory and renal compensations.
The release of the ketone bodies into the blood results in a metabolic acidosis. As in the case of any metabolic acidosis, there is an increase in the rate of breathing in order to expel CO2. In addition, the kidney produces NH3 from glutamine via glutaminase and from glutamate via glutamate dehydrogenase (GDH) in order to increase the excretion of protons (NH3 + H+ → NH4+).
Glutamine is coming from branch-chain amino acid (BCAA) degradation in muscle. The amount of glutamine going to kidney is increased as the gut, typically a major consumer of glutamine, is now using KB.
How (and where) are ketone bodies utilized?
β-hydroxybutyrate is oxidized to acetoacetate. The acetoacetate then is linked to CoA supplied by succinyl CoA via acetoacetate: succinyl-CoA CoA transferase (absent in liver). The acetoacetyl CoA produced is cleaved to 2 acetyl CoA. The process occurs in the mitochondrial matrix of peripheral tissues, and is of special importance in brain.
What would be the basis for concluding that the protein breakdown and amino acid release in skeletal muscle is due to the hypoinsulinemia and not the result of elevated glucagon levels?
Skeletal muscle does not have glucagon receptors
What role might this protein degradation play in the functioning of the liver under these conditions?
Protein degradation in the muscle results in the release of amino acids into the blood, primarily gln and ala. These are the major substrates for gluconeogenesis in the liver.
Ala (produced from succinyl CoA generated in the catabolism of the BCAA, val and ile) travels in the blood to liver where it is transaminated to pyruvate that can be used as a substrate for gluconeogenesis.
Transamination also produces glu that can be oxidatively deaminated to α-KG + NH3. Gln, also produced by BCAA catabolism, travels to liver (and kidney) and is converted first to glu + NH3 by glutaminase and then to α-ketoglutarate + NH3 by GDH.
The α-ketoglutarate can be used for gluconeogenesis, and the NH3 is detoxified to urea. Hepatic metabolism of ala and gln generates NH3 that is detoxified in the urea cycle: ala + gln carry NH3 to the liver.
How would diabetic conditions affect fatty acid and triglyceride synthesis in adipose tissue?
The adipose tissue synthesizes fatty acids and triglycerides from glucose. As the adipose tissue expresses the insulin-dependent glucose transporter, in conditions of hypoinsulinemia glucose uptake would be limited.
