How Well Do You Understand Blood Glucose Levels?

Vijay Aswani, PhD

March 10, 2010

Introduction

Why does practically every lab physician include an order for blood glucose determination?

Except under conditions of starvation, the brain depends exclusively on glucose as its source of energy (other tissues can use fatty acids and ketone bodies as alternate sources of fuel).

Red blood cells also depend exclusively on glucose as their source of energy, since they do not have the mitochondria necessary to extract energy from fatty acids, ketone bodies, or oxidative phosphorylation.

The following clinical case and questions focus on testing an understanding of various biochemical factors that affect blood glucose levels. The cases discuss the key processes that are involved in that which might affect glucose levels clinically, such as:

  • Alcohol consumption and gluconeogenesis

  • Hormones: insulin, glucagon, epinephrine

  • Gluconeogenesis enzyme-related disorders:

    -Glucose 6-phosphatase
    -PEP carboxykinase
    -Pyruvate kinase
    -Fructose 1,6-bisphosphatase

  • Glycolysis enzyme-related disorders:

    -Pyruvate kinase

  • Glycogen-related diseases:

    -McArdle's
    -Von Gierke's
    -Pompe's

  • Respiratory control, muscular exertion, glycogenolysis, and glycolysis

The following set of questions is devised to refresh your understanding of the biochemical underpinnings of blood glucose.

Case

A 45-year-old man of Asian Indian origin presents at his family physician's office for an annual checkup. Given that the patient's father and grandfather suffered from type 2 diabetes mellitus, the physician recommends that the patient undergo a test for the condition. He prescribes a glucose tolerance curve and asks the patient to report to the lab the next morning, fasting.

The next morning, a blood sample is withdrawn from the patient and he is given 75 mg of dextrose to drink. Over the next several hours, blood samples are collected from the patient at several intervals. The results of the glucose estimation by the GOD/POD method are shown in .

  Table 1. Glucose Estimation by GOD/POD

Time mg/dL
Fasting 89
30 minutes 123
1 hour 145
2 hours 91
3 hours 83
4 hours 90

Answer the following questions based on these data:

Why did the blood glucose levels fall steadily over the 4-hour period after ingestion of the dextrose sample?

  1. There was a rapid consumption of glucose by all of the tissues in the patient's body; he had not eaten since the previous night

  2. Most of the glucose was pulled out of circulation by the muscle where it was converted to glycogen

  3. The increased blood glucose traveling the bloodstream stimulated the pancreatic beta cells to release insulin that caused increased uptake of glucose by liver, muscle, and adipose tissue

  4. The glucose was consumed by red blood cells which broke it down to CO2 and H2O through glycolysis, the Krebs cycle, and oxidative phosphorylation

  5. The brain had been deprived of this vital fuel during the prolonged fast and rapidly consumed the glucose

View the correct answer.

Discussion

Glucose phosphorylated by the enzyme glucokinase.

While it is true that the patient had not eaten since the previous night, this is not the reason for the fall in blood glucose. When glucose enters the bloodstream during a feeding period, about 60% of it is phosphorylated by the enzyme glucokinase in the liver (Figure 1).

This enzyme is found only in the liver. It is not allosterically inhibited by feedback inhibition like the glycolytic enzyme hexokinase.

The liver is the major organ involved in the homeostasis of blood glucose. Through processes like glycogenolysis and gluconeogenesis, it matches the consumption of glucose by organs and tissues such as the brain and red blood cells, thus keeping blood glucose levels within the normal range. The activities of these metabolic processes, in turn, are under the control of hormones such as insulin and glucagon. The release of these hormones is dependent on blood glucose levels.

Therefore, how long the patient has gone without eating does not have a direct bearing on blood glucose levels.

However, when blood glucose levels in the body are raised beyond their normal range (70 gm/dL to 120 mg/dL), the increased glucose levels bind to receptors on beta cells in the islet of the pancreas and stimulate the release of insulin.

This insulin releases trigger effects of its own:

  • It increases the number of insulin-dependent glucose transport proteins (GLUT) in the membranes of adipocytes and skeletal muscle. This facilitates their being able to absorb glucose from the bloodstream.

  • It inhibits the release of glucagon from the alpha cells of the islet of Langerhans

  • It stimulates protein synthesis of key enzymes involved in glycolysis in the liver: glucokinase, phosphofructokinase-1 and pyruvate kinase. These, in turn, facilitate greater absorption and trapping of glucose by the liver as glucose 6-phosphate.

  • The binding of glucose to its receptor on the cells of the liver and other tissues leads to intracellular changes that result in:

    -Dephosphorylation and activation of glycogen synthase in the liver, leading to the synthesis and replenishing of liver glycogen reserves

    -Dephosphorylation and inactivation of glycogen phosphorylase in the liver, preventing the breakdown of liver glycogen to maintain glucose

    -Dephosphorylation and activation of the domain phosphofructokinase-2, an enzyme that catalyzes the formation of fructose 2,6-bisphosphate -- a key allosteric activator of phosphofructokinase-1, the rate-limiting step of glycolysis

    -Dephosphorylation and inactivation of fructose 2,6-bisphosphatase -- the enzyme that would break down fructose 2,6-bisphosphate -- the key allosteric inhibitor of Fructose 1,6-bisphosphatase, the rate-limiting step of gluconeogenesis

Question 2

Even thought the patient had last eaten at 9:00 pm the previous night, his blood glucose levels were still within normal range the following morning at the start of the test. What mechanisms were responsible for this?

  1. Glucagon levels in his bloodstream would rise a couple or so hours after his last meal that night and this hormone's action maintained his blood glucose levels in homeostasis

  2. Glycogen reserves in his liver were being broken down and the resultant glucose formed was being used to maintain blood glucose levels

  3. Although various organs and systems such as his brain and red blood cells continued to consume glucose at a steady state during the night, blood glucose was being replenished by the liver and kidneys

  4. Some skeletal muscle protein was broken down and the resultant amino acids were converted to glucose by the liver

  5. All of the above

View the correct answer.

Discussion

Mechanisms by which blood glucose levels are prevented from going below levels that could lead to hypoglycemia include:

  • A fall in blood glucose decreases insulin levels in the bloodstream. This leads to a release in glucagon by the alpha cells of the islets of Langerhans. Glucagon, a counterregulatory hormone, opposes the actions of insulin described previously.

  • The binding of glucagon to receptors on liver cells leads to a series of intracellular events mediated through a G-protein and cAMP-mediated secondary messenger system. A key result is the phosphorylation of various key enzymes involved in glucose generation and consumption:

    -The enzyme glycogen phosphorylase is phosphorylated and activated, thus switching on the process of breaking down liver glycogen ultimately to glucose, which the liver delivers into the bloodstream.

    -The PFK-2 domain is phosphorylated and inactivated, shutting down the formation of fructose 2,6-bisphosphate. This compound inhibited the rate-limiting step of gluconeogenesis -- Fructose 1,6-bisphosphatase. Gluconeogenesis is thus switched on in the liver and the kidneys. Some skeletal muscle proteins are broken down to amino acids. This leads to increased levels of alanine and glutamine, among others in the bloodstream. These amino acids are absorbed by the liver and converted to glucose through deamination and gluconeogenesis.

Question 3

Which of the following organs is the most important in maintaining glucose homeostasis?

  1. Brain

  2. Liver

  3. Kidney

  4. Adipocytes

  5. Skeletal muscle

View the correct answer.

Discussion

There are several key biochemical processes within cells associated with either the consumption or generation of blood glucose. See if you can name the rate-limiting steps of these processes.

Question 4

Rate-limiting step of glycolysis is catalyzed by:

  1. Glucokinase

  2. Pyruvate kinase

  3. Phosphofructokinase-1

  4. Fructose 1,6-bisphosphatase

  5. Glucose 6-phosphatase

View the correct answer.

Question 5

The rate-limiting step of glycogen breakdown (glycogenolysis) is catalyzed by:

  1. Glycogen synthase

  2. Glucose 6-phosphatase

  3. Glycogen phosphorylase

  4. Glycogen phosphorylase kinase

  5. Phosphoglucomutase

View the correct answer.

Question 6

The rate-limiting step of glycogenesis (glycogen synthesis) is catalyzed by:

  1. Alpha-glucosidase

  2. Alpha-glucosidase

  3. Hexokinase

  4. Glycogen synthase

  5. Glycogen phosphorylase

View the correct answer.

Question 7

The rate-limiting step of gluconeogenesis is catalyzed by:

  1. Pyruvate carboxylase

  2. PEP carboxykinase

  3. Glucokinase

  4. Glucose 6-phosphatase

  5. Fructose 1,6-bisphosphatase

View the correct answer.

Discussion

provides the answers to rate-limiting steps.

  Table 2. Table of Enzymes

Enzyme Reaction Pathway Comments
Glucokinase Glucose + ATP Glucose 6-phosphate + ADP Glycolysis Found in liver only; higher activity at higher concentrations of glucose
Pyruvate kinase PEP + ADP Pyruvate + ATP Glycolysis Catalyzes last step of glycolysis; irreversible reaction. Enzyme activated by feed forward action fructose 1,6-bisphosphate; synthesis stimulated by insulin.
phosphofructokinase-1 (PFK-1) Fructose 6-phosphate + ATP Fructose 1, 6-bisphosphate + ADP Glycolysis Rate-limiting step of glycolysis; allosteric activation by fructose 2,6-bisphosphate produced by PFK-2, an enzyme activated by insulin
fructose 1,6-bisphosphatase (FBP-1) Fructose 1,6-bisphosphate Fructose 6-phosphate + Pi Gluconeogenesis Rate-limiting step of gluconeogenesis; allosterically inhibited by fructose 2,6-bisphosphate broken down by FBP-2, an enzyme activated by glucagon
glucose 6-phosphatase Glucose 6-phosphate Glucose + Pi Gluconeogenesis Catalyzes the last step in gluconeogenesis; irreversible reaction
glycogen synthase UDP-Glucose + Glycogen Glycogen + PPi Glycogenesis Rate-limiting step of glycogen synthesis; activated by insulin through dephosphorylation; inactivated by glucagon through phosphorylation; adds glucose units to existing glycogen but cannot build glycogen from scratch
glycogen phosphorylase Glycogen (Glucose)n + Pi Glycogen (Glucose)(n-1) + Glucose-1-phosphate Glycogenolysis Rate-limiting step of glycogen breakdown; activated by glucagon and epinephrine through phosphorylation
glycogen phosphorylase kinase Glycogen phosphorylase + ATP Glycogen phosphorylase -- P + ADP Glycogenolysis regulation in muscles This enzyme converts the inactive (dephosphorylated) form of glycogen phosphorylase to the active (phosphorylated) form
phosphoglucomutase Glucose-1-phosphate <—> Glucose-6-phosphate Part of glycogenolysis Important enzyme that forms glucose 6-phosphate from the breakdown of glycogen. Glucose 6-phosphate can then be used in glycolysis or converted to glucose by the action of glucose 6-phosphatase.

Question 8

An unconscious patient is brought to the ER by his friends. They explained that after working on a 20-hour shift they went to a local bar to celebrate. The patient had 3 drinks and acted quite intoxicated. Normally, his friends said, he could drink much more than that. Blood tests reveal not very high levels of alcohol but severe hypoglycemia (< 40 mg/dL). Furthermore, a mild acidosis was noted (pH 7.35). What connection could the alcohol have to the hypoglycemia?

  1. Alcohol breakdown led to an inhibition of glycogen breakdown

  2. Alcohol intake or breakdown had nothing to do with the hypoglycemia

  3. Alcohol intake led to increased utilization of glucose by the liver, lowering the blood glucose levels

  4. Alcohol breakdown prevented gluconeogenesis from occurring, thus preventing maintenance of glucose levels in normal range

  5. Alcohol breakdown led to formation of acetic acid, which destroys glucose

View the correct answer.

Discussion

As noted, alcohol is broken down by the enzyme alcohol dehydrogenase (Figure 2).

Alcohol breakdown.

This by itself should not matter. However, after glycogen reserves in the liver are depleted, blood glucose is maintained through gluconeogenesis. Two crucial first steps in the synthesis of glucose from pyruvate involve availability of intermediates that are swayed by a high NADH concentration. Viewing the pathway (Figure 3), one can see that under conditions of high NADH concentrations, pyruvate is converted to lactate, by lactate dehydrogenase (LDH), and oxaloacetate is converted to malate by malate dehydrogenase (MDH), thus converting key gluconeogenetic intermediates into reduced forms and blocking gluconeogenesis.

Under conditions of high NADH concentrations, pyruvate is converted to lactate by LDH, and oxaloacetate is converted to malate by MDH.