What Does Renin Breakdown Protein Into? A Scientific Clarification

November 27, 2025 •

4 min read

The similarity in names causes frequent confusion, but renin and rennin are two entirely different enzymes serving distinct purposes in the body. Renin, which originates in the kidneys, is part of a complex system for regulating blood pressure, while rennin is primarily a digestive enzyme active in the stomachs of infants and young mammals. This article will clarify exactly what does renin breakdown protein into, contrasting it with the function of rennin.

Quick Summary

Renin, an enzyme secreted by the kidneys, breaks down the liver protein angiotensinogen into angiotensin I to initiate blood pressure regulation. The similarly named enzyme rennin, found in infants, digests the milk protein caseinogen into insoluble casein or curds.

Key Points

Renin vs. Rennin: Renin and rennin are distinct enzymes; renin is for blood pressure regulation (renal system), while rennin is for milk digestion (digestive system).
Renin's Target: Renin breaks down the protein angiotensinogen, which is produced in the liver, into angiotensin I.
Rennin's Target: Rennin (chymosin) breaks down the milk protein caseinogen into insoluble casein, causing milk to curdle.
Location of Function: Renin is secreted by the kidneys into the bloodstream, whereas rennin acts in the stomachs of young mammals.
Human Digestion: Adults produce little to no rennin and digest milk using pepsin and other enzymes, unlike infants and young animals.
RAAS Initiation: Renin's action is the crucial first step in the renin-angiotensin-aldosterone system (RAAS), which controls blood pressure.
Commercial Use: Rennin is commercially used as 'rennet' in cheesemaking to curdle milk.

The question, "What does renin breakdown protein into?" often stems from a common misconception involving two distinct enzymes with very similar names: renin (with one 'n') and rennin (with two 'n's). To provide a complete and accurate answer, it is necessary to examine both enzymes and their specific, non-interchangeable functions within the body.

Renin: The Kidney Enzyme

Renin is a crucial enzyme produced and secreted by the juxtaglomerular cells in the kidneys. Its primary role is not general protein digestion but rather a highly specific proteolytic action within the complex Renin-Angiotensin-Aldosterone System (RAAS). The RAAS is a hormonal system that plays a critical role in regulating blood pressure and fluid balance in the body.

The Renin-Angiotensin-Aldosterone System (RAAS)

When blood pressure or blood volume decreases, the kidneys are signaled to release renin into the bloodstream. Renin then acts as a highly specific endopeptidase, targeting only one particular protein: angiotensinogen.

Angiotensinogen: This is a large, precursor protein produced by the liver and constantly circulating in the blood.
Cleavage: Renin cleaves angiotensinogen, separating a 10-amino acid peptide chain called angiotensin I.
Further Conversion: Angiotensin I is biologically inactive but is subsequently converted into the potent vasoconstrictor, angiotensin II, by another enzyme called angiotensin-converting enzyme (ACE). Angiotensin II then triggers a cascade of effects to increase blood pressure, including blood vessel constriction and the release of aldosterone, a hormone that promotes sodium and water retention.

Therefore, to be precise, renin breaks down the protein angiotensinogen into angiotensin I, initiating the process that controls blood pressure.

Rennin: The Digestive Enzyme

Rennin, also known as chymosin, is a protein-digesting enzyme found in the gastric juices of infants and young mammals, particularly ruminants like cows. Unlike renin, its function is directly related to food digestion within the stomach.

The Action of Rennin on Milk Protein

In young, milk-fed animals, rennin is secreted to help process milk protein. It acts specifically on the milk protein caseinogen, a soluble form of casein.

Coagulation: Rennin converts the soluble caseinogen into insoluble casein in the presence of calcium ions. This process is known as curdling.
Curd Formation: The resulting insoluble casein forms a solid curd within the stomach. This allows the milk to stay in the stomach longer, exposing the protein to other digestive enzymes like pepsin for a more extended period and leading to more efficient digestion and nutrient absorption.
Human Infants: While rennin is crucial for milk digestion in young mammals, humans generally produce little to no rennin. Instead, human infants and adults rely on pepsin and other enzymes to digest milk proteins.

Comparison: Renin vs. Rennin

The key to understanding the distinction lies in their roles, locations, and target proteins. The following table summarizes their main differences:


Feature	Renin (Kidney Enzyme)	Rennin (Digestive Enzyme)
Primary Location	Kidneys	Stomach lining of infants/young mammals
Main Function	Initiates blood pressure regulation (RAAS)	Curdles milk protein (casein)
Target Protein	Angiotensinogen	Caseinogen
End Product	Angiotensin I (cleaved from angiotensinogen)	Insoluble casein (curds)
Presence in Adults	Yes, part of the RAAS	Largely absent in humans
System Involved	Cardiovascular and Renal System	Digestive System

How Adults Digest Milk Proteins

Since adult humans have little to no rennin, the task of digesting milk protein falls to other proteolytic enzymes. In the stomach, pepsin—which is activated by stomach acid—is the main enzyme responsible for breaking down proteins into smaller peptides. Pepsin is also capable of coagulating milk, though less specifically than rennin. These smaller peptides are then further broken down into individual amino acids by enzymes in the small intestine, such as trypsin and chymotrypsin.

Conclusion: Separating the Two Enzymes

In summary, asking "What does renin breakdown protein into?" is a question with two parts due to the similarly named enzymes. The correct answer depends on which enzyme is being referred to. The kidney-produced renin breaks down the protein angiotensinogen into angiotensin I to regulate blood pressure and volume. In contrast, the stomach-based rennin (or chymosin) breaks down the milk protein caseinogen into insoluble casein for digestive purposes in young mammals. Understanding this distinction is key to comprehending these two very different biological processes. For more information on the renin-angiotensin-aldosterone system, you can visit the Cleveland Clinic's article on RAAS.

Components of the Renin-Angiotensin-Aldosterone System (RAAS):

Renin: The enzyme released by the kidneys that begins the process.
Angiotensinogen: The protein precursor produced by the liver that renin acts upon.
Angiotensin I: An inactive peptide resulting from renin's cleavage of angiotensinogen.
Angiotensin-Converting Enzyme (ACE): An enzyme, primarily in the lungs, that converts Angiotensin I into its active form.
Angiotensin II: A potent hormone that causes blood vessel constriction and triggers aldosterone release.
Aldosterone: A hormone from the adrenal glands that promotes sodium and water retention, increasing blood volume.

Steps in Rennin-Mediated Milk Digestion:

Prorennin Secretion: The inactive precursor is released into the stomach.
Activation: Hydrochloric acid in the stomach activates prorennin into active rennin.
Caseinogen Cleavage: Rennin specifically cleaves the milk protein caseinogen.
Curdling: In the presence of calcium ions, the resulting insoluble paracasein coagulates, forming curds.
Further Digestion: The curds are retained in the stomach longer, allowing other enzymes like pepsin to break them down more efficiently.

Understanding the distinction between renin and rennin is vital for anyone interested in human and animal physiology. While renin influences the entire body's blood pressure, rennin’s role is localized to the digestion of a specific food source in infancy.

Frequently Asked Questions

Yes, renin is found in humans. It is an enzyme secreted by the kidneys and is a crucial part of the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure.

No, human adults do not produce significant amounts of rennin. The gene for rennin is largely inactivated in the primate lineage, and adults rely on the enzyme pepsin for milk protein digestion.

Angiotensinogen is a large protein precursor produced primarily by the liver. It circulates in the blood and is the specific protein substrate that the enzyme renin cleaves.

Rennin is important for young mammals because it coagulates the milk in their stomachs. This slows the rate at which milk passes through the digestive system, allowing for more complete digestion and absorption of nutrients by other enzymes like pepsin.

Rennin, often referred to as rennet in this context, is used in cheesemaking to cause milk to separate into solid curds and liquid whey. Its curdling action is the foundational step for producing most types of cheese.

Humans digest milk protein primarily using the stomach enzyme pepsin. Pepsin, like rennin, is a proteolytic enzyme that breaks down proteins, including milk's casein, into smaller peptides for further digestion.

The immediate end product of renin's action on angiotensinogen is the decapeptide called angiotensin I. Angiotensin I is then converted into the more active angiotensin II by another enzyme.

After rennin curdles milk by converting soluble caseinogen into solid curds, other digestive enzymes, such as pepsin, can more effectively act upon the protein. This slows the digestive process and improves nutrient absorption.