Enzymes and hormones

Salivary amylase

• Source: Salivary glands

• Inactive form: None (active)

• Activation: Already active

• Stimulus: Parasympathetic (cephalic phase, food in mouth)

• Site of action: Oral cavity (continues briefly in stomach)

• Optimum pH: ~6–7

• Substrate: Starch

• Action: Breaks α-1,4 bonds

• Products: Maltose, short polysaccharides

pepsin

• Source: Chief cells (as pepsinogen)

• Activation:

  • By HCl in stomach

  • Also autocatalytic

• Stimulus: Gastrin, vagus nerve

• Site of action: Stomach lumen

• Optimum pH: ~2

• Substrate: Proteins

• Action: Endopeptidase (cuts internal bonds)

• Products: Smaller peptides

gastric lipase

• Source: Chief cells

• Inactive form: Active

• Stimulus: Vagal stimulation

• Site of action: Stomach

• Optimum pH: Acidic (~4–5)

• Substrate: Triglycerides

• Action: Removes fatty acids

• Products: Diglycerides + fatty acids

trypsin

• Source: Pancreas (trypsinogen)

• Activation:

  • By enterokinase (brush border)

• Stimulus: CCK, vagus

• Site: Small intestine

• pH: ~7–8

• Substrate: Proteins

• Action: Endopeptidase

• Products: Peptides

chymotrypsin

Secreting cell/organ: pancreas

Inactivated version: Chymotrypsinogen

How and where activated: Trypsin

Stim for release:

Site of action: Small intestine

Optimum pH:

Substrate: proteins

Action: Endopeptidase

Products: peptides

carboxypeptidase

Secreting cell/organ:pancreas

Inactivated version: Procarboxypeptidase

How and where activated: Trypsin

Stim for release:

Site of action: Small intestine

Optimum pH:

Substrate: Peptides

Action: exopeptidase

Products:Amino acids

Pancreatic lipase

• Source: Pancreas

• Activation: Active, but requires colipase

• Stimulus: CCK

• Site: Small intestine

• Optimum pH: ~7–8

• Substrate: Triglycerides

• Action: Removes 2 fatty acids

• Products: Monoglycerides + free fatty acids

Pancreatic amylase

• Source: Pancreas

• Activation: Active

• Stimulus: CCK, vagus

• Site: Small intestine

• Optimum pH: ~7

• Substrate: Starch

• Action: Breaks α-1,4 bonds

• Products: Disaccharides, oligosaccharides

colipase

• Source: Pancreas

• Inactive form: Procolipase

• Activation: By trypsin in small intestine

• Stimulus: CCK, vagus

• Site of action: Small intestine

• Optimum pH: ~7–8

• Substrate: N/A (cofactor, not enzyme activity itself)

• Action: Anchors pancreatic lipase to fat droplets (overcomes bile inhibition)

• Products: Enables triglyceride digestion

phospholipase

• Source: Pancreas

• Activation: By trypsin

• Stimulus: CCK

• Site: Small intestine

• Optimum pH: ~7–8

• Substrate: Phospholipids

• Action: Cleaves fatty acid

• Products: Lysophospholipids + fatty acids

enterokinase

• Secreting cell/organ: Intestinal epithelial cells (duodenum)

• Inactivated version: None (active)

• Activation (how/where): Already active on brush border

• Stimulus for release: Presence of chyme (especially proteins)

• Site of action: Brush border of small intestine

• Optimum pH: ~6–8 (neutral)

• Substrate: Trypsinogen

• Action: Cleaves and activates trypsinogen

• Products: Trypsin

• Brush border enzyme? YES

maltase

• Secreting cell/organ: Intestinal epithelial cells

• Inactivated version: None

• Activation: Already active

• Stimulus: Presence of maltose

• Site of action: Brush border

• Optimum pH: ~6–7

• Substrate: Maltose

• Action: Hydrolyzes disaccharide

• Products: 2 glucose molecules

• Brush border enzyme? YES

sucrase

• Secreting cell/organ: Intestinal epithelial cells

• Inactivated version: None

• Activation: Already active

• Stimulus: Presence of sucrose

• Site of action: Brush border

• Optimum pH: ~6–7

• Substrate: Sucrose

• Action: Splits disaccharide

• Products: Glucose + fructose

• Brush border enzyme? YES

lactase

• Secreting cell/organ: Intestinal epithelial cells

• Inactivated version: None

• Activation: Already active

• Stimulus: Presence of lactose

• Site of action: Brush border

• Optimum pH: ~6

• Substrate: Lactose

• Action: Hydrolyzes disaccharide

• Products: Glucose + galactose

• Brush border enzyme? YES

aminopeptidase

• Secreting cell/organ: Intestinal epithelial cells

• Inactivated version: None

• Activation: Already active

• Stimulus: Presence of peptides

• Site of action: Brush border

• Optimum pH: ~7–8

• Substrate: Small peptides

• Action: Removes amino acids from N-terminal end (exopeptidase)

• Products: Free amino acids

• Brush border enzyme? YES

gastrin

Stim for release: peptides and amino acids; neural reflexes

Primary target: ECL cells and parietal cells

primary effect: Stimulates gastric acid secretion and mucosal growth

Other: Somatostatin inhibits release

CCK

Stim for release: Fatty acids and some amino acids

Primary target: Gallbladder, pancreas, stomach

primary effect: stimulates gallbladder contraction and pancreatic enzyme secretion; inhibits gastric emptying and acid secretion.

other: promotes satiety; some effects may be due to CCK as a neurotransmitter

secretin

• Stimulus: Acid in duodenum

• Target: Pancreas

• Effect: ↑ bicarbonate

motilin

Stim for release: Fasting: periodic release every 1.5-2 hours

Primary target: gastric and intestinal smooth muscle

primary effect: stimulates migrating motor complex

other: inhibited by eating a meal

GIP

Stim for release: Glucose, fatty acids, and amino acids in small intestine

Primary target: Beta cells of pancreas

primary effect:Stimulates insulin release(feedforward mechanism); inhibits gastric emptying and acid secretion

GLP-1

Stim for release: Mixed meal that includes carbohydrates or fats in teh lumen

Primary target: Endocrine pancreas

primary effect: stimulates insulin release; inhibits glucagon release and gastric function

other: promotes satiety

HCL

• Source: Parietal cells

• Stimulus: Gastrin, histamine, vagus

• Function:

  • Activate pepsin

  • Denature proteins

  • Kill bacteria

• Site: Stomach lumen

mucus

• Source: Mucous cells

• Stimulus: Irritation, vagal input

• Function: Protection

• Site: GI lining

bicarbonate

• Source: Pancreas, duodenum

• Stimulus: Secretin

• Function: Neutralize acid

• Site: Small intestine

intrinsic factor

• Source: Parietal cells

• Function: B12 absorption

• Site of action: Ileum

histamine

• Source: ECL cells

• Stimulus: Gastrin

• Function: ↑ HCl secretion (paracrine)

bile

• Source: Liver (stored in gallbladder)

• Stimulus: CCK

• Function: Emulsifies fats

• Site: Small intestine