GASTROINTESTINAL PHYSIOLOGYChapter-II

(DIGESTION and ABSORPTION)Ass. Prof. Dr. Emre Hamurtekin

EMU Faculty of Pharmacy

INTRODUCTION

• CARBOHYDRATES• PROTEINS• NUCLEIC ACIDS• LIPIDS• VITAMINS & MINERALS

CARBOHYDRATES• The principal dietary carbohydrates are:

– Polysaccharides (Starches)• Amylopectin***• Amylose

– Disaccharides (lactose & sucrose)

– Monosaccharides (fructose & glucose)

• Salivary and pancreatic α-amylases hydrolyze 1:4 α bonds.• The end products of α-amylase digestion are:

– Maltose– Maltotriose– α- limit dextrin– Glucose oligomers

• Oligosaccharidases are located in the brush border of small intestinal epithelial cells.

• Isomaltase is responsible for hydrolysis of 1:6 α bonds

CARBOHYDRATES

glucose fructose

SucraseSucrose

LactoseLactase

glucose galactose

CARBOHYDRATES

• SGLT-1 transports glucose & galactose into the intestinal epithelial cell.

• Transport of glucose and galactose is dependent on Na in the intestinal lumen.

• Fructose absorption is independent of Na; it is transported by facilitated diffusion.

• GLUT 5 and GLUT 2 is responsible for the absorption of fructose.

PROTEINS

• Protein digestion begins in the stomach (pepsins).

• Pepsins are secreted as proenzymes and activated by gastric acid.

• Pepsinogen I• Pepsinogen II• Products of peptic digestion are polypeptides.

PROTEINS

• Polypeptides are further digested in the small intestine by pancreatic and intestinal proteolytic enzymes.

• Endopeptidases:– Trypsin– Chymotrypsin– Elastase

• Exopeptidases:– Carboxypeptidase A– Carboxypeptidase B

PROTEINS

PROTEINS

PROTEINS

• Brush border of the intestinal mucosal cell:– Aminopeptidases– Carboxypeptidases– Endopeptidases– Dipeptidases

• Intracellular peptidases hydrolyzes some di-and tripeptides that enters into the intestinal epithelium.

PROTEINS

• At least seven different transport systems transport a.a. into enterocytes.

• Five of them cotransport a.a. and Na.• Two of them are Na independent.• Di- and tripeptides are transported into

enterocytes by PepT1 system (requires H).• A.a absorption is rapid in duodenum and

jejunum.

NUCLEIC ACIDSPancreatic nucleases

nucleotides

nucleosides phosphoric acid

Intestinal enzymes

sugars purine / pyrimidine bases active transport

blood

LIPIDS• Most of the fat digestion begins in the duodenum (pancreatic lipase*)• The products are:

– FFA– 2-monoglycerides

• Pancreatic lipase acts on emulsified fats and activitiy is facilitated by colipase

LIPIDS

• Steatorrhea:– Fatty, bulky stool because of the impaired

digestion and absorption of the fat.– Mostly due to the lipase deficiency,– Gastric acid hypersecretion & defective

reabsorption of bile salts are the other causes.– Malabsorption of fat-soluble vitamins results.

LIPIDS

• Transport of lipids into the enterocytes occurs mainly by passive diffusion, but also carriers may be involved.

FA < 10-12 C atomsno modification

FA

ENTEROCYTE

FFA

BLOOD

FA > 10-12 C atomsesterified

TG

active transport

+ ChE

ChylomicronsLYMPHATIC CIRCULATION

Chylomicrons

exocytosis

VITAMINS

• Fat-soluble vitamins: Vitamin A, D, E and K

Esterase

Incorporation into micelles

Absorption

Most vitamins

Upper small intestine

absorbed

Vit B12

Ileum

VITAMINS

• Vit B12 and folate absorption are Na-independent.• Remaining water-soluble vitamins – Thiamin– Riboflavin– Niacin– Pyridoxine– Pantothenate– Biotin– Ascorbic acid

Na cotransporters

THE END