Which pancreatic enzyme is secreted in an active form?

Pancreatic Enzyme Secretion: Active vs. Inactive Forms

Lipase is the only pancreatic enzyme that is secreted in an active form, while trypsin, phospholipase A, and elastase are all secreted as inactive precursors (zymogens) that require activation in the intestinal lumen. 1, 2

Pancreatic Enzyme Secretion Physiology

• The pancreas secretes more than 10 different enzymes along with water, bicarbonate, and other proteins (such as enzyme inhibitors) that are essential for proper digestion 2
• Most pancreatic enzymes are secreted as inactive precursors (zymogens) to prevent autodigestion of the pancreas itself 2
• Trypsinogen is the inactive precursor (zymogen) of trypsin, which requires activation in the intestinal lumen 2
• Phospholipase A and elastase are also secreted as inactive precursors that require activation by trypsin in the duodenum 2
• Lipase is unique among pancreatic enzymes as it is secreted in its active form and does not require activation 1, 3

Characteristics of Individual Pancreatic Enzymes

Lipase

• Secreted in active form directly from the pancreas 1, 3
• Functions to hydrolyze triglycerides into diglycerides and subsequently into monoglycerides and free fatty acids 4
• Remains elevated in serum for 8-14 days after pancreatic injury, making it a more reliable diagnostic marker than amylase 2, 1
• Requires colipase (another pancreatic secretion) for full activity in the intestinal lumen 5

Trypsin

• Secreted as trypsinogen, an inactive precursor 2
• Activated in the duodenum by enterokinase (enteropeptidase), an enzyme produced by the duodenal mucosa 2
• Once activated, trypsin activates other pancreatic zymogens in a cascade reaction 2
• Serum trypsinogen levels rise within hours of pancreatic injury and decrease within 3 days 2

Phospholipase A

• Secreted as prophospholipase A, an inactive precursor 2
• Requires activation by trypsin in the intestinal lumen 2
• Hydrolyzes phospholipids in dietary fat 2

Elastase

• Secreted as proelastase, an inactive precursor 2
• Activated by trypsin in the intestinal lumen 2
• Can be measured in feces as a marker of pancreatic function 2

Clinical Significance

• The secretion of most pancreatic enzymes as inactive precursors is a protective mechanism to prevent autodigestion of the pancreas 2
• In acute pancreatitis, premature activation of trypsinogen within the pancreas initiates a cascade of enzyme activation leading to pancreatic autodigestion and inflammation 2
• Serum lipase is considered more reliable than amylase for diagnosing acute pancreatitis due to its higher sensitivity (79% vs 72%) and longer elevation period 2, 1
• Measurement of pancreatic enzymes in serum and feces can be used to assess pancreatic function and diagnose pancreatic disorders 2, 1

Common Pitfalls in Clinical Practice

• Assuming all pancreatic enzymes require activation can lead to misunderstanding of pancreatic physiology and pathophysiology 2, 1
• Failing to recognize that lipase remains elevated longer than amylase may lead to misinterpretation of laboratory results in suspected pancreatic disease 2, 1
• Not considering that non-pancreatic conditions can also cause elevation of pancreatic enzymes, particularly amylase 1, 6