3. Carbohydrates Flashcards
Carbohydrates: Function
Need ATP for:
Molecular Synthesis
Muscle Contraction
Active Transport of Nutrients into Cells
What happens when the daily dietary energy intake exceeds daily expenditure?
The excess is converted to fat and is stored in adipose tissue
Dietary Sources: Starches
- Wheat
- Rice
- Potatoes
Dietary Sources: Glucose/Fructose
- Molasses
- Fruits
- Honey
Dietary Sources: Lactose
Milk products
What is the storage form of glucose in animals? What is its primary storage site?
Glycogen is stored primarily in the liver; stored glycogen can be used to raise blood glucose levels
Muscle Glycogen
- Muscle glycogen makes up 2/3 of glycogen mass
- Can’t be used to raise blood glucose levels because muscle lacks the enzyme: Glucose-6-phosphatase
Classification of Carbohydrates: First Classification
Based on the number of sugar units in a chain
Monosaccharides
- Simple sugars that can’t be hydrolyzed to a simpler form
- Subunits from which the other groups are formed
- Sugars containing 3, 4, 5, 6 or more carbon atoms
- Colorless, crystalline solids that are water-soluble
e. g. glucose, galactose, fructose, mannose
Notation of Monosaccharides
Carbohydrates are hydrates of either an aldehyde or a ketone group
Glyceraldehyde
- Aldose
- Carbonyl Group is at the end of the chain
Fischer Projection Model: D-Glucose
e. g. D-Glucose
* * Look at the OH group on the highest numbered symmetrical carbon
* * If on the right, it is the D Form.
Fischer Projection Model: L-Glucose
L-Glucose
- The OH group on the highest asymmetrical carbon is on the Left Side
- It is the L Form
Stereoisomers: Definition
These are compounds that are identical in composition and differ only in spatial configuration
- D Glucose is the most common form
Beta vs. Alpha
Look at the Carbonyl Group:
If it is on the Left, we call it Beta
If it is on the Right, we call it alpha
Haworth Projection Model
Most Monosaccharides with 5 or more carbons usually occur in aqueous solutions as cyclic or ring structures in which the carbonyl group has formed a covalent bond with the oxygen of the hydroxyl group along the chain
When converting from the Fischer model to the Haworth model the following rules apply:
- All groups to the right in the Fischer Model will be written downward.
- All groups on the left in the Fischer Model will be written upward.
Chemical Properties of Monosaccharides: Oxidation vs. Reduction
Monosaccharides can be oxidized by Ferric (Fe3+) or Cupric (Cu2+) ions.
- To be oxidized is to lose an electron
- To be reduced is to gain an electron
Reducing Sugars: Defintion
Sugars capable of “reducing” (gaining electrons) Ferric or Cupric ions
Disaccharides
Two monosaccharides joined together
e.g. maltose = glucose + glucose
Formation of Disaccharides: Condensation Reaction
- Two monosaccharides are joined together when the hydroxyl group on one sugar reacts with the carbonyl carbon of the other
- Referred to as a condensation reaction because water is generated
Hydrolysis: Definition
When Disaccharides are broken down, Hydrolysis Occurs (Loss of Water)
Why are some sugars not considered reducing sugars?
When the carbonyl carbon is part of the glycosidic bond, it cannot be oxidized
The end of the chain that has the free carbonyl carbon is the reducing end
What does it mean when both Carbonyl carbons are involved in the bond?
If both Carbonyl carbons are involved in the bond, the sugar cannot be a reducing agent
Polysaccharides: Definition
Linkage of many monosaccharide units together
Amylose: Linkage
Only alpha 1,4, linkages
Amylopectin: Linkage
Alpha 1, 6 Linkage branch every 25 glucose units
Glycogen (animal storage): Linkage
Alpha 1,6 Linkage branch every 8-12 glucose units
** More compact than starch types
Metabolism of Glucose: Salivary Amylase
Salivary Amylase hydrolyzes these nonabsorbable forms into Dextrins and Disaccharides
What happens after salivary amylase is produced?
These products now go to the stomach
The stomach pH inhibits Salivary Amylase
Pancreatic Amylase in the small intestines will hydrolyze these products into maltose, sucrose, and lactose
Digestive Enzymes will be released by the..
Intestinal Mucosa
Digestive Enzymes: Maltase
Breaks down Maltose into two glucose monosaccharides
Digestive Enzymes: Sucrase
Breaks down Sucrose into glucose and fructose
Digestive Enzymes: Lactase
Breaks down Lactose into glucose and galactose monosaccharides
Glucose, Galactose, and Fructose are absorbed into the blood supply and taken to the..
LIVER
- Glucose is the only one that can be used directly for energy or storage
- Galactose and Fructose must be converted into glucose before they can be used
Glucose can be:
- Stored in the liver as glycogen
- Metabolized to CO2 + H2O for immediate energy within a cell
- Converted to Keto Acids or Amino Acids
- Converted to Fat and stored in adipose tissue
- Released into circulation
Steps of Glycolysis
Glucose –> Hexokinase –> Glucose-6-P
Glucose-6-Phosphate –> Fructose-6-P
Fructose-6-Phosphate- -> Fructose1, 6 Diphosphate
Glyceraldehyde-3P and Dihydroxyacetone-P are produced
Glycolysis: Where does it take place? How many ATP are produced?
Formation of Pyruvate from Glucose
- Made a total of 4 ATP molecules
- Used 2 ATP molecules
- Net gain of 2 ATP molecules per molecule of glucose
- Glycolysis takes place in the cytoplasm of the cell
What happens when ATP is high?
When ATP is high, the rate Acetyl CoA enters the citric acid cycle decreases and the synthesis of Fatty Acids increase
Glycolysis: End Results
Produces 4 pairs of electrons (in the form of NADH and FADH2)
These electrons pass through the respiratory chain which ultimately drives the synthesis of ATP
Net Effect: 38 Molecules* of ATP are generated from the glycolysis of glucose
Other names for Citric Acid Cycle (Aerobic Process)
- Tricarboxylic Acid
- Krebs Cycle
Galactose Feeder Pathway
Galactose must be converted into glucose to be utilized
Galactose
Galactose-1-Phosphate
Glucose-1-Phosphate
Glucose- 6-Phosphate –> Glycolysis
Gluconeogenesis: Definition & Purpose
- Formation of Glucose from non-carbohydrate sources
- Maintains glucose levels during starvation and/or vigorous exercise
- Occurs primarily in the Liver
- Use Amino Acids from the break-down of Protein or Glycerol from the break-down of fat
How many ATP molecules are used to make glucose?
Have to spend 6 molecules of ATP to make Glucose
Source of Carbohydrate Fasting Individual
- Liver Glycogen
- Carbohydrate found in Plasma: Glucose
- Fast lasts longer than one day (Gluconeogenesis)
Glucose –6-Phosphatase
Converts Glucose -6 Phosphate back into glucose; Found in Liver but not in muscle
Hexokinase
Converts Glucose to Glucose-6-Phosphate
Glycogen Phosphorylase
Degrades glycogen by breaking the alpha 1,4 linkages to release glucose units. Also adds a phosphate group that keeps glucose from diffusing out of the cell
Pancreas: Insulin
- Synthesized by the β Cells of the Islet of Langerhans
- Stimulates movement of glucose into cells
- Increased glucose utilization (glycolysis)
- Increases hepatic and muscle glycogenesis
- Increases Lipogenesis (Fat)
- Inhibits gluconeogenesis
- Inhibits glycogenolysis
Name the only hormone that decreases blood glucose levels, hypoglycemic agent
Insulin
Glucagon
- Primary hormone responsible for increasing blood glucose levels
- Hyperglycemic agent
- Synthesized by the alpha cells of the islets of Langerhans
- Stimulates glycogenolysis in the liver increases Lipolysis and Gluconeogenesis
Somatostatin
- Produced by the Delta (δ) Cells of the islets of Langerhans
- Inhibits the secretion of Insulin and Glucagon,
- Helps modulate the reciprocal relationship between the two hormones
- Increases plasma glucose
Anterior Pituitary Gland
- Growth Hormone
- Inhibits the uptake of glucose by cells
- Insulin inhibitor
ACTH (Adrenocorticotropic Hormone)
Causes the release of Cortisol from the adrenal cortex which: increases glycogenolysis in the liver increases gluconeogenesis Lipolysis Net Effect: Increase Plasma Glucose
Adrenal Medulla: Epinephrine
- Secreted in the time of stress
- Stimulates glycogenolysis and lipolysis
- Inhibits insulin secretion
Net Effect: Increases Plasma Glucose
Thyroid Gland
Thyroxine (T4)
- Stimulates glycogenolysis, gluconeogenesis.
- Increases intestinal absorption of glucose
Net Effect: increases plasma glucose
Hyperglycemia
Abnormally high levels of glucose
- associated with Diabetes Mellitus
Type I Diabetes
Insulin Dependent Diabetes Mellitus
- Absolute Deficiency of Insulin
- Makes up 10-20% of all patients with Diabetes Mellitus
- Primarily diagnosed in childhood and adolescence
- Autoimmune destruction of the Beta Cells of the Pancreas
- 85-90% of patients demonstrate autoantibodies to islets cells or Insulin
- Usually requires Insulin therapy
- Associated with ketoacidosis
Type I Diabetes: Complications
- neuropathies
- cataracts
- nephropathies
- atherosclerosis
- vascular insufficiency
Type I Diabetes: Symptoms
(1) polydipsia: excessive thirst
(2) polyphagia: increased food intake
(3) polyuria: excessive urine production rapid weight loss
Type II Diabetes Mellitus
- Non-Insulin Dependent Diabetes Mellitus
- Most common form of Diabetes Mellitus
- 80-90% of patients with Diabetes Mellitus
Diagnosed usually after 40 years old. - Insulin levels are higher than normal.
- There is a resistance to insulin due to a decrease in insulin receptor sites in the obese patient
- Usually not associated with Ketoacidosis
Type II Diabetes: Treatment
(1st) Weight Reduction for Obese patients
(2nd) Dietary Restrictions
(3rd) Tolbutamide (oral hypoglycemic agent)
(4th) Small doses of Insulin
Gestational Diabetes Mellitus
- Abnormal Glucose concentration during pregnancy
- Patients frequently return to normal after delivery
- 30% probability that women with GDM will develop Diabetes Mellitus within 20 years
- Greater risk if delivered a baby greater than 9 lbs.
- Infants born to GDM mothers have an increased risk of respiratory distress syndrome, hypocalcemia, and hyperbilirubinemia
Secondary Causes of Diabetes Mellitus
- Chronic Pancreatitis
- Drugs/ Chemical Induced Diabetes (Dilantin)
- Endocrinopathies
- Hyperthyroidism
- Hyper-pituitarism
When any one of the following criteria are met and confirmed on a subsequent day, the diagnosis of Diabetes is confirmed:
- Random plasma glucose >200 mg/dL and symptoms of diabetes
- Fasting plasma glucose >126 mg/dL
- Two hour plasma glucose > 200 mg/dL during an OGTT
- Hgb A1C: > 6.5% (certified method)
** American Diabetes Association (ADA) recommends a Fasting Blood Sugar or Hgb A1C test for everyone over the age of 45 years old every 3 years. People in high risk groups are recommended to have it more often.
Relative Hypoglycemia
- Normal fasting glucose level
- Occurs in relationship to a meal
Functional Hypoglycemia
- Common in adults
- Occurs 2-4 hours after eating a meal
Prediabetic or Diabetic Hypoglycemia
Insulin response is delayed and exaggerated
Alimentary Hypoglycemia
- Patients who have had G.I. surgery
- Accelerated absorption of glucose- yields an exaggerated insulin response
Drug-Induced Hypoglycemia
Fasting glucose is low
(1) Alcohol: Has a negative effect on glycogen storage; glycolysis
(2) Insulin
(3) Tolbutamide or oral hypoglycemic agents
Insulinoma
Beta Cell Tumors
- 80% are benign
- 10% are malignant
- 10% are multiple tumors
Endocrinopathies
- Hypothyroidism
- Hypopituitarism
- Addisons Disease
- Adrenal Cortical Insufficiency: decreased Cortisol
Massive Liver Disease: Definition
Glycogen Storage Disease
- Deficiency in Glucose-6-Phosphatase, therefore cannot convert glycogen to glucose
- Also called Von Gierke Disease
- Enlarged liver due to increased glycogen stores
- Severe hyperlipidemia
- Capillary Blood: higher than venous blood
Massive Liver Disease: Reference Ranges
Plasma: <100 mg/dL Whole Blood: 65 – 95 mg/dL CSF: 40 – 70 mg/dL *Approximately 60% of Plasma Glucose Levels *When blood glucose levels change, it takes 2 hours for CSF/blood glucose equilibrium to occur
Specimens
Stability
- Serum on Cells: decreased glucose (5 –7% per hour)
- Plasma should be separated from cells within 1 hour
- 8 hours at room temperature
- 72 hours at 4C
Preservatives
- Sodium Fluoride/ Iodoacetate gives Plasma
- Prevents RBCs, WBCs, and PLTs from metabolizing glucose (inhibits glycolytic enzymes)
- Stable for 3 days at room temperature
Glucose Oxidase: What does it measure?
Polarographic Oxygen Electrode Method (CX3)
- The electrode measures the Rate of Oxygen Consumption after the addition of the glucose sample to a buffered solution containing glucose oxidase reagent
- The greater the glucose concentration in the sample - the faster the Rate of Oxygen Consumption
Glucose Oxidase Method- Chromogenic (Trinder Method)
This method uses the same initial reaction as above with the addition of a chromogen which is oxidized by peroxidase to give a colored reaction
- The amount of colored product formed (oxidized chromogen) is proportional to the glucose concentration in the sample
- The Glucose Oxidase method is specific for B-D-Glucose
- Mutarotase can be added to convert -D-Glucose (36% of glucose) into B-D-Glucose
- The incubation time will be longer for the test
Chromogenic (Trinder Method): Interferences
- Chromogen can be influenced by uric acid, ascorbic acid, and bilirubin.
- These substances are oxidized by peroxidase and will decrease the amount of chromogen oxidized giving a falsely low glucose result
- Bleach can oxidize the chromogen and give a falsely elevated glucose result
- Urine samples cannot be used for this method due to the presence of enzyme inhibitors in urine
Hexokinase Method
Glucose + ATP = Glucose-6-Phosphate + ADP (G-6-P)
G-6-PD: Yeast vs. Bacteria Sources
G-6-PD from yeast sources use NADP+ as the co-factor
G-6-PD from bacteria sources use NAD+ as the co-factor
** Not affected by interferences like Glucose Oxidase
Whole Blood Test Strip Methods
- Glucometers are Reflectance Meters that use Glucose Oxidase-Peroxidase Chromogen test strips
- Reflectance Meter: The colored reaction produced by the test is measured by light that is reflected as compared to reflected light from a reference surface
- This is not a linear reaction like the spectrophotometer. There is a microprocessor that uses math algorithms to calculate the concentration of glucose
- Whole blood glucose levels are 11% less than serum or plasma values
- These methods are primarily used by diabetic patients to monitor their own blood glucose levels for insulin therapy-3-4 timers per day
2 –Hour Post Prandial Test
- Patient is given a 75 gram dose of glucose
- Two hours later, blood is drawn
- If the level is > 200 mg/dL, and is confirmed on another day, the patient is diagnosed with Diabetes Mellitus
- Less than 140 mg/dL is considered normal
Oral Glucose Tolerance Test
- Not recommended for routine use as the first screening test
- The serial measurement of plasma glucose before and after glucose is given orally
When a OGTT is ordered, the following
conditions must be met:
- Diet containing >150 g of carbohydrates for 3 days prior to the test
- Fasting 10-16 hours before the test
- Fasting specimen must be below 126 mg/dL before the Glucola can be given
- The oral glucose load is 75 grams for adults and 1.75 g/kg, up to 75 grams maximum for children
- Should be ingested over 5 minutes
- The Glucose Tolerance Test would measure Plasma glucose at fasting, and then every 30 minutes for 2 hours
Interfering Substances
Drugs and Medications:
- Oral contraceptives
- Salicylates
- Nicotinic acid (cigarettes, cigars, etc)
- Diuretics (caffeine)
- Hypoglycemic Agents (Insulin, etc)
- Anticonvulsants (Dilantin)
- Corticosteroids
Time of Day
Morning due to diurnal variation in glucose values
The OGTT test is best used to assess:
- Borderline Fasting glucose levels
- Risk counseling (High Risk for developing Diabetes)
- Diagnosis of Gestational Diabetes
Universal Screening for Gestational Diabetes Mellitus:
- High Risk Patients should be screened for GDM.
- Risk Factors include:
- Older than 25 years of age
- Overweight
- Family History of Diabetes
- Glycosuria
One-Step Glucose Tolerance
- 2 hour OGTT using a 75 g Glucose Load
- Gestational Diabetes if:
- Fasting: >92 mg/dL
- 1 Hour: > 180 mg/dL
- 2 Hour: > 153 mg/dL
2-Step Glucose Tolerance: STEP 1
Step 1:
50 g Glucose Load is given.
If > 140 mg/dL after one hour then proceed to Step 2
2-Step Glucose Tolerance: STEP 2
Step 2:
- 100 g Glucose Load
- 3 hour OGTT
- Gestational Diabetes if:
- *Fasting: >92 mg/dL
- *1 Hour: > 180 mg/dL
- *2 Hour: > 153 mg/dL
- *3 Hour: > 140 mg/dL
All who are positive for Gestational Diabetes will be:
- Screened again 6-12 weeks postpartum
- Life Long screen every 3 years
Glycosylated Hemoglobin
- Want to maintain control over glucose levels when managing Diabetes.
- `By measuring Glycosylated Hemoglobin, the physician can see long term compliance with monitoring and achieving blood glucose level control
Normal Adult Type Hemoglobin
Hemoglobin: A1 97% - 2 alpha chains; 2 beta chains Hemoglobin: A2 2.5% - 2 alpha chains; 2 delta chains Hemoglobin F: 0.5% - 2 alpha chains; 2 gamma chains
Circulating normal adult Hemoglobin A1 is made up of
A number of minor hemoglobins- Hgb A1a, Hgb A1b, Hgb A1c and are referred to as the glycohemoglobins
Hgb A1c
- Makes up 80% of Hemoglobin A1
- Glucose molecule attaches to one or both ends (N-terminal Valine) of the Beta chain of normal adult Hemoglobin
- Nonenzymatic reaction
The rate of formation is proportional to:
The amount of plasma glucose concentration
Irreversible, so because the average red cell lives 120 days, the Glycosylated Hemoglobin level reflects:
Plasma glucose levels over the previous 2-3 months unrelated to short-term fluctuations in plasma glucose levels
- Normal Values: 3.0 – 6.0%
- Estimated Average Glucose (EAG) can be calculated:
- 7 X A1c – 46.7
- Recommended that A1c be reported out with a mean glucose level
- Goal is to maintain levels of Hemoglobin A1c around 7.0% for Diabetics
Interferences: Red Blood Cell Life Span decreased due
to disease such as..
(1) Hemoglobinopathies
(2) Hemolytic anemia, etc;
* * The hemoglobin will have less time to become glycosylated and levels will be falsely decreased
* * Longer Life span of Red Cells (post-splenectomy) may have increased levels
Abnormal Hemoglobins in Homozygous State
If there is no Hemoglobin A1 present, there cannot be Hgb A1c
HOWEVER:
Variant Hemoglobin’s can be glycosylated—but are NOT linked to certified methods and cannot be correlated in the same way!
- Hemoglobin’s S and C have substitutions closer to the N-terminal of the beta chain, therefore interfere in some methods
- Hemoglobin’s E and D have substitutions further away –so are less likely to interfere in some methods
Methods for A1C
(1) Based on charge differences between glycosylated and non-glycosylated hemoglobin’s
e. g. Cation-Exchange High Pressure Liquid Chromatography (HPLC), Electrophoresis
(2) Those based on structural differences between glyco-groups
e. g. Immunoassays, Affinity Chromatography
High Pressure Liquid Chromatography Cation Exchange
- Bio-Rad Variant II Turbo
- Hemoglobin’s are separated based upon their charge in a cation-exchange column.
Stationary Phase
- The sample is injected into a cartridge which contains negatively charged binding sites (Stationary Phase)
- Positively charged hemoglobins will bind to those sites
- Some hemoglobin variants have more “positively charged” sites and will bind tighter than other hemoglobin variants
Mobile Phase
Three Phosphate buffers of increasing ionic strength are used to elute the individual components which are read on a spectrophotometer at 415 and 690 nm (Mobile Phase)
Interferences with this method:
(1) Temperature Dependent
(2) Hemoglobin F >25%
Immunoassay
- Siemens Advia 1650
- Roche Cobas is similar only turbidimetric
- Latex Agglutination inhibition
- Normally, the agglutinator (synthetic Hb A1c) will bind to reagent antibody (Anti-Hb A1c) that is attached to a latex bead
Agglutination produces an increase in..
Scattered light which is read by the instrument
Patient Hb A1c when present, will compete for the binding sites on the antibody..
Preventing the agglutinator from binding
- Produces decreased light scattering
- Patient Hb A1c has an inversely proportional relationship with light scattering
Affinity Chromatography
- Glycosylated Hemoglobin attaches to the resin but non-glycosylated hemoglobin does not
- The glycosylated hemoglobin is then removed from the resin bed using a buffer
- Not affected by Temperature
Specimen Requirement for A1c
EDTA Whole Blood Specimen Hemolysate is prepared Patients should have this test: Twice a year: Controlled Diabetics Quarterly: Treatment Changes Uncontrolled Diabetics
Fructosamine (Plasma Protein Ketoamine)
- Glycated Albumin and Protein
- Albumin has a half-life of 2-3 weeks
- Glucose Control over 2-3 weeks
- Responds more quickly to changes in therapy
- Fructosamine reduces Nitro-Blue Tetrazolium to Formazan
- (Colorimetric at 530 nm)
Microalbuminuria
- Useful in the diagnosis of early kidney disease in diabetic patients
- Type I Diabetics are typically screened annually beginning 5 years after diagnosis
- Type 2 Diabetics screened annually from diagnosis
How is microalbuminuria assessed?
Microalbuminuria is assessed by using the Albumin to Creatinine Ratio
Present when the Albumin – Creatinine Ratio:
30 – 299 mg of Albumin to 1 gm of Creatinine
Specimen Requirement for Microalbumin
- Random Urine (Most Common)
- Positive result in 2 out of 3 urine samples collected within a period of 3 -6 months as recommended by the ADA
False Increases in Urinary Albumin
- Exercise within 24-hour period
- Infection
- Fever
- Congestive Heart Failure
- High Blood Pressure
C-Peptide
Part of the Proinsulin Molecule
What removes the C-peptide during purifiation? What is this process useful for?
- Commercial insulin removes the C-peptide during purification
- Useful in distinguishing between endogenous and exogenous insulin
- Helps diagnose Insulinomas
- An Insulinoma is an insulin- secreting tumor; Most are benign.
Urinary Sugars: How and why is it examined?
Urine is examined routinely to detect the presence or determine the amount of glucose
Glucose Renal Threshold: 170 mg/dL
Urinary Sugars: Definition
The plasma concentration of glucose above which glucose cannot be reabsorbed by the renal tubules and thus is excreted into the urine
Galactosemia: Definition
Inability to metabolize galactose
Deficiency: Galactose-1-Phosphate Uridylyl Transferase; Type 1 GALT Gene Mutation
Galactosemia: Complications
- Liver disease
- Cataracts
- Mental Retardation
- Ecoli or other gram negative sepsis
Lactose Intolerance Testing: Hydrogen Breath Test
- Normally have very little Hydrogen or Methane in breath
- If unable to break down lactose, it will be fermented in the colon by bacteria releasing hydrogen and methane gases
- Patient takes a baseline breath test
- Drink a liquid that contains a source of lactose
- Takes Breath samples at 15 minute intervals over 2-3 hours
- The breath is measured for Hydrogen, Methane, and Carbon Dioxide
