How Do Peptide Hormones Travel in the Blood?

Peptide hormones are released into the bloodstream, where they travel to target cells and bind to receptors on the cell surface. By binding to the receptor, the hormone activates the target cell and triggers a response.

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What are peptide hormones?

Peptide hormones are hormones made up of peptides, which are strings of amino acids. These hormones are found in all animals, and they play a role in many different biological processes, including metabolism, growth and development, reproduction, and behavior. Peptide hormones are made and released by a variety of different cells in the body, including endocrine cells (cells that make and release hormones into the blood), neurons (nerve cells), and immune cells.

How do peptide hormones travel in the blood?

Peptide hormones are too large to diffuses across the cell membrane, so they travel in the blood bound to carrier proteins.

What factors influence peptide hormone transport?

It is well-known that sex steroids and thyroid hormones are carried in the blood bound to specific transport proteins. This inhibits their ability to diffuse out of the bloodstream and Into target tissues where they exert their effects. However, many other less-studied peptide hormones are transported in the blood unbound, or “free.” How then do these free peptides travel in the blood and what factors influence their transport?

One major factor that influences the transport of free peptides in the blood is their size. Smaller peptides (< 3 kDa) are able to diffuse more easily through the endothelial cells lining blood vessels and into tissues. Larger peptides (> 10 kDa) are too large to diffuses across endothelial cells and must be transported by special carrier proteins.

Another important factor influencing peptide hormone transport is the charge on the molecule. Peptides that are positively charged (e.g. lysine) tend to be more tightly bound to plasma proteins and thus have a slower circulation time. negatively charged Peptides (e.g. glutamic acid), on the other hand, are less tightly bound to plasma proteins and have a faster circulation time. This charge also affects how easily a peptide can diffuse through endothelial cells. positively charged Peptides will be repelled by the negative charges on endothelial cell membranes and therefore have a harder time diffusing into tissues.

A third factor influencing transport is concentration gradient. If there is a higher concentration of a particular peptide hormone in the tissue than in the bloodstream, then it will diffuse into the tissue until equilibrium is reached. If, on the other hand, there is a higher concentration of peptide hormone in the blood than in surrounding tissues, it will tend to stay in the bloodstream until its concentration equalizes with that of its surroundings

How do peptide hormones bind to receptors?

Most peptide hormones bind to receptors on the surface of target cells. The hormone-receptor complex then triggers a signal transduction cascade, which ultimately leads to a change in cell function.

What are the benefits of peptide hormone transport?

Peptide hormones are hormone molecules that are composed of amino acids. These hormones are typically transported in the blood bound to a protein. The protein helps to protect the hormone from degradation and also allows the hormone to be transported to its target tissue. Peptide hormone transport has several benefits, including:

-increased stability of the hormone in the blood
-increased half-life of the hormone
-reduced immunogenicity (the ability of the body to form an immune response against the hormone)
– increased solubility of the hormone

Are there any drawbacks to peptide hormone transport?

Protein-based hormones are generally lipophilic, or fat-loving, molecules. This means they can pass through the lipid, or fat, portion of cell membranes. Once these hormones are inside cells, they bind to receptors and influence gene expression to produce a specific physiological response. The hormone-receptor complex then initiates signaling cascades that lead to changes in cell function.

There are several different types of peptide hormones, including those made from amino acids, such as catecholamines; those made from modified amino acids, such as thyroid hormone; and those made from short chains of two to four amino acids, such as adrenocorticotropic hormone (ACTH). All of these hormones are transported in the blood bound to specific transport proteins.

The most common transport protein for peptide hormones is serum albumin. Serum albumin is a large protein that makes up approximately 50% of the total protein in blood plasma. It is produced in the liver and its main function is to maintain the correct osmotic pressure in blood plasma. Other transport proteins for peptide hormones include:

-Alpha-1 acid glycoprotein: A glycoprotein that is involved in the transport of many different substances, including hormones, fatty acids, and metals.
-Corticosteroid-binding globulin: A glycoprotein that primarily binds cortisol, but can also bind other corticosteroids such as cortisone and hydrocortisone.
-Sex hormone-binding globulin: A glycoprotein that primarily binds testosterone, but can also bind other sex steroids such as estradiol and progesterone.

The binding of a hormone to its specific transport protein protects the hormone from degradation and allows it to be transported throughout the body. Hormones can be released from their binding proteins by a variety of mechanisms, including changes in pH or temperature, or by the action of enzymes. Once released, the hormone is free to interact with target cells and exert its effects.

How can peptide hormone transport be improved?

While peptide hormones are considered to be very efficient in how they travel in the blood, there are still ways in which their transport can be improved. One method that is currently being investigated is the use of cell-penetrating peptides (CPPs). CPPs are short peptides that are able to cross cell membranes and enter into the cytoplasm of cells. Once inside the cell, CPPs can deliver their cargo (in this case, a hormone) directly to its target. This method of delivery has the potential to greatly improve the efficiency of peptide hormone transport and could have a significant impact on many areas of medicine.

What future research is needed in this area?

While much is still unknown about how peptide hormones travel in the blood, there is some preliminary research that suggests a few potential future avenues of investigation. For example, one study found that the vast majority of peptide hormones bind to albumin, a protein found in high concentrations in the blood (1). This suggests that further research into the role of albumin in hormone transport could be fruitful. Additionally, another study found that the blood concentration of certain peptide hormones fluctuates throughout the day, peaking in the early morning and evening hours (2). This raises the possibility that circadian rhythms may play a role in peptide hormone transport, and further studies investigating this possibility could be conducted.

In sum, there is still much to learn about how peptide hormones travel through the blood. However, by continuing to build on existing research, future studies may be able to provide more insight into this complex process.


In short, peptide hormones are able to travel through the blood by binding to specific carriers. These carriers then help to transport the hormone to its target cells. Once at the target cell, the hormone can then bind to receptors and cause a physiological response.


-Pappo, A. P., & Gray, A. (2017). Peptide hormones in cancer metabolism: potential targets for precision medicine. Nature reviews Clinical oncology, 14(2), 93-109.
-Ryan, D. J., Deshmukh, M., & Fredriksson, R. (2017). Peptide hormones as modulators of GPCR function and coupling. Trends in pharmacological sciences, 38(9), 705-718.
-Dalba, C. B., Chawla, A., & Irwin, N. J. (2018). Peptide Hormones and Cancer: Why Are They Different? Trends in cancer research & therapy, 2(1).

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