Clinical diagnostic biochemistry -1 Dr. Maha Al-Sedik 2015 CLS 334

Carbohydrates, including sugar and starch, are widely distributed in plants and animals. They perform multiple functions, ranging from being structural components of deoxyribonucleic acid (DNA) and ribonu­cleic acid (RNA) (ribose and deoxyribose sugars) to serve as sources of energy (glucose). What is carbohydrates?

Definition The term carbohydrate refers to hydrates of carbon (one molecule of water per carbon atom). IntroductionIntroduction Glucose is derived from: (1) The breakdown of carbohydrates in the diet or in body stores (glycogen). (2) Endogenous synthesis from protein or from triglycerides.

When caloric intake exceeds needs: the excess is converted to fat for storage in adipose tissue and glycogen for storage in liver. When energy needs exceeds caloric intake: Endogenous glucose formation from the breakdown of carbohydrate stores and from non - carbohydrate sources.

Classification Monosaccharide: Monosaccharides, or simple sugars, consist of a single sugar unit and cannot be hydrolyzed to a simpler form. Sugars containing three, four, five, six, and seven carbon atoms are known as trioses, tetroses, pentoses, hexoses, and heptoses respectively.. Disaccharides: Two monosaccharides join covalently with the loss of a molecule of water, to form a disaccharide, so it can be hydrolyzed to simpler form.

The most common disaccharides are: Maltose = glucose + glucose Lactose = glucose + galactose Sucrose = glucose + fructose The most common disaccharides are: Maltose = glucose + glucose Lactose = glucose + galactose Sucrose = glucose + fructose سلم

Polysaccharides: The linkage of multiple monosaccharide units results in the formation of polysaccharides. The major storage carbohydrates are starch in plants and glycogen in animals, both of which form granules inside cells.

Story of glucose After a meal, carbohydrate is broken down to glucose, absorbed into the bloodstream, and carried to the body’s cells. On seeing, the glucose level in blood, pancreas secret’s insulin that help the cell to consume glucose for its energy.

Excess of glucose from food is converted into glycogen or fat and stored in liver or fat cells. The body can use glycogen for energy between meals. Fat can also be used for energy.

When blood-glucose begins to fall, glucagon signals the liver to break down glycogen and release glucose into the bloodstream. Glucose level in blood will then rise toward a normal level.

Regulation of Blood Glucose Concentration Glycogenesis: Is the synthesis of glycogen from glucose. Glycogenolysis: Is the reverse process, the break-down of glycogen to glucose and other intermediate products. Gluconeogenesis: Is the formation of glucose from non carbohydrate sources, such as amino acids, glycerol, or lactate. Glycolysis: Is the conversion of glucose or other hexoses into lactate or pyruvate.

Hormones that regulate blood glucose concentratio n

 Insulin is a protein hormone produced by the (β-cells of the islets of langerhans in the pancreas.  It is an anabolic hormone that stimulates the uptake of glucose into fat and muscle.  It promotes the conversion of glucose to glycogen or fat for storage.  It inhibits glucose production by the liver.  It stimulates protein synthesis.  It inhibits protein breakdown. INSULIN

Actions of insulin

Insulin is the only hypoglycemic hormone in the body

1.Pre-pro-insulin, a protein of about 100 amino acids (MW 12,000), is formed by ribosomes in the rough endoplasmic reticulum of the pancreatic β –cells. 2.Pre-pro-insulin is not detectable in the circulation under normal conditions because it is rapidly converted by cleaving enzymes to pro-insulin (MW 9000). 3.Pro insulin is a 86-amino acid polypeptide. It is stored in secretory granules in the Golgi complex of the β -cells, where proteolytic cleavage it to insulin ( 51 a.a ) and connecting peptide (C-peptide) (31 a.a ). Synthesis of insulin:

Pre-pro insulin Pro insulin Insulin C-peptide

Release of insulin: Factors stimulate insulin release:  Hyperglycaemia.  Some medications (e.g., Sulfonylureas & β -adrenergic agonists). Factors inhibit insulin release:  Hypoglycemia.  Some medication: (e.g., Α-adrenergic agonists, β -adrenergic blockers).

Degradation of insulin: On the first pass through portal circulation, approximately 50% of the insulin is extracted by the liver, where it is degraded. Additional insulin degradation occurs in the kidneys. The half life insulin in the circulation is between 5 and 10 minutes.

 Proinsulin is cleaved to a 31-amino acid con­necting (C) peptide (MW 3600) and insulin.  Although insulin and C-peptide are secreted into the portal circulation in equal amounts, fasting C-peptide con­centrations are 5 – 10 folds higher than those of insulin owing to the longer half-life of C-peptide (about 35 minutes).  The liver does not extract C-peptide, which is removed from the circulation by the kidneys and degraded, with a fraction excreted unchanged in the urine. C- PEPTIDE

C peptideInsulin 35 min5 – 10 minHalf life 5 – 10 folds more than insulin lowFasting level kidneyLiver then kidney Degradation

Counter-regulatory hormones:  Glucagon & adrenaline are rapid-acting counter-regulatory hormones.  growth hormone & cortisol are released, When hypoglycemia is prolonged.

I N S U LI N  Glucagon  Adrenaline  Growth hormone  Cortisol  Thyroxine

gluconeogenesisglycogenolysislypolysisproteolysis glycolysisglycogenesisLipid synthesis Uptake of glucose by muscle and respiratory muscle COUNTER REGULATORY↑ Insulin ↑

Insulin  Evaluation of fasting hypoglycemia.  Classification of diabetes mellitus.  Select optimal therapy for diabetes.  Diagnosis of B- cell tumor. Glucagon  Diagnosis of α-cell tumors. Clinical Utility of Measuring Insulin, Proinsulin, C-Peptide, and Glucagon

ProInsulin  Diagnosis of B-cell tumors  Familial hyperproinsulinemia C-Peptide  Evaluation of fasting hypoglycemia.  Classification of diabetes mellitus.  Select optimal therapy for diabetes.  Diagnosis of B- cell tumor.

Hyperglycemia Increase in the blood glucose. Causes : 1- Insulin deficiency e.g. Diabetes mellitus. 2- Increased anti-insulin hormones e.g. adrenaline, thyroxine, growth hormone and glucocorticoids. 3- Excessive food intake. 4- Emotional stress. Decreased insulin Increased counter regulatory hormones

HYPOGLYCEMIA Hypoglycemia is a blood glucose concentration below the fasting value, but it is difficult to define a specific limit. The most widely suggested cut off is 50 mg/dL transient decline may occur 1.5 to 2 hours after a meal.

Causes : 1- Insulin overdose. 2- Increase in hypoglycemic medications. 3- Insulinoma. 4- Decreased counter regulatory hormones. 5- Malnutrition and prolonged starvation. Increased insulin or hypoglycemic drug. Decreased counter regulatory hormones. Hypoglycemia

Symptoms of hypoglycemia:  Mode changes.  Sweating.  Nausea.  Hunger.  Very low concentrations of plasma glucose (less than 20 or 30mg/dL) cause severe CNS dysfunction.

 Immediate consumption of 15 gm of simple form of carbohydrate  2 or 3 glucose tablets (must be easily chewable)  1/2 cup of any fruit juice  pieces of hard candy  Measure after 15 minutes.  Followed by snack of carbohydrate foods e.g. biscuit or fruits or milk to maintain normoglycaemia until the next meal or snack. Treatment of mild to moderate hypoglycemia ( role of 15 /15 ):

 Many people like the idea of treating hypoglycemia with cake, cookies, and brownies.  However, sugar in the form of complex carbohydrates or sugar combined with fat and protein are much too slowly absorbed to be useful in the acute treatment of hypoglycemia Important note

 Hypoglycemia: sweating.  Hyperglycaemia: dry skin.

GLUCOSURIA Presence of glucose in urine when blood glucose level exceeds renal threshold (> 180 mg / dl). Causes : 1- D.M. 2- Excessive intake of carbohydrates. 3- Renal glucosuria (congenital reduction of renal threshold). 4- Nephritis and nephrosis.

Reference: Burtis and Ashwood Saunders, Teitz fundamentals of Clinical Chemistry, 4th edition, 2000.