Serotonin, also known as 5-hydroxytryptamine (5-HT), is produced and/or stored in various locations within the body (refer to Fig. 21.18). The largest quantity can be found in the intestinal mucosa, while smaller amounts are present in the central nervous system (CNS) where it serves as a neurotransmitter, as well as in platelets (refer to online Chapter 35). Serotonin is synthesized from tryptophan, which undergoes hydroxylation in a reaction that requires BH4, similar to the process catalyzed by phenylalanine hydroxylase. The resulting product, 5-hydroxytryptophan, is then decarboxylated to form 5HT. Serotonin plays multiple physiological roles including the perception of pain and the regulation of sleep, appetite, temperature, blood pressure, cognitive functions, and mood (inducing a sense of well-being). Selective serotonin reuptake inhibitors (SSRIs) are utilized as antidepressants as they help maintain serotonin levels. Serotonin is broken down by MAO into 5-hydroxy-3-indoleacetic acid (5-HIAA).

Creatine phosphate, also referred to as phosphocreatine, is the phosphorylated derivative of creatine found in muscles. It is a high-energy compound that provides a small yet rapidly mobilized reserve of high-energy phosphates. These phosphates can be transferred reversibly to adenosine diphosphate (refer to Fig. 21.19) in order to sustain the intracellular level of ATP during the initial few minutes of intense muscular contraction. The amount of creatine phosphate present in the body is proportionate to the muscle mass. Creatine is synthesized in the liver and kidneys using glycine, the guanidino group of arginine, and a methyl group from SAM (refer to Fig. 21.19). Animal products serve as dietary sources of creatine. Creatine is enzymatically phosphorylated to creatine phosphate by creatine kinase, utilizing ATP as the phosphate donor. Creatine and creatine phosphate undergo spontaneous cyclization at a slow but constant rate, resulting in the formation of creatinine. Creatinine is excreted in the urine and its quantity is proportional to the total content of creatine phosphate in the body. This allows for the estimation of muscle mass. A decrease in muscle mass, such as from paralysis or muscular dystrophy, leads to a decrease in urinary creatinine content. Additionally, an increase in blood creatinine serves as a sensitive indicator of kidney dysfunction since creatinine is typically rapidly cleared from the blood and excreted. An average adult male excretes approximately 1-2 g of creatinine per day.

Melanin is a pigment present in various tissues, particularly the eye, hair, and skin. Its synthesis occurs in melanocytes, which are pigment-producing cells located in the epidermis. Melanin serves the purpose of safeguarding underlying cells against the harmful effects of sunlight. A deficiency in melanin production leads to oculocutaneous albinism, with the most common type being caused by defects in copper-containing tyrosinase (refer to p. 273).

CHAPTER SUMMARY: 
- Serotonin, also known as 5-hydroxytryptamine (5-HT), is primarily found in the intestinal mucosa but also exists in smaller amounts in the CNS and platelets. It is synthesized from tryptophan and has various physiological roles.
- Creatine phosphate is a high-energy compound present in muscles that aids in maintaining ATP levels during intense muscular contraction. It is synthesized in the liver and kidneys and is used as an indicator of muscle mass and kidney function.
- Melanin is a pigment synthesized in melanocytes that protects cells from sunlight. A deficiency in melanin production results in oculocutaneous albinism.
Serotonin, also called 5-hydroxytryptamine (5-HT), is synthesized and/or stored at several sites in the body (Fig. 21.18). The largest amount by far is found in the intestinal mucosa. Smaller amounts occur in the CNS, where it functions as a neurotransmitter, and in platelets (see online Chapter 35). Serotonin is synthesized from tryptophan, which is hydroxylated in a BH4 requiring reaction analogous to that catalyzed by phenylalanine hydroxylase. The product, 5-hydroxytryptophan, is decarboxylated to 5HT. Serotonin has multiple physiologic roles including pain perception and regulation of sleep, appetite, temperature, blood pressure, cognitive functions, and mood (causes a feeling of well-being). [Note: Selective serotonin reuptake inhibitors (SSRI) maintain serotonin levels, thereby functioning as antidepressants.] Serotonin is degraded by MAO to 5hydroxy-3-indoleacetic acid (5-HIAA).
D. Creatine
Creatine phosphate (also called phosphocreatine), the phosphorylated derivative of creatine found in muscle, is a high-energy compound that provides a small but rapidly mobilized reserve of high-energy phosphates that can be reversibly transferred to adenosine diphosphate (Fig. 21.19) to maintain the intracellular level of ATP during the first few minutes of intense muscular contraction. [Note: The amount of creatine phosphate in the body is proportional to the muscle mass.] 1.
Synthesis: Creatine is synthesized in the liver and kidneys from glycine and the guanidino group of arginine, plus a methyl group from SAM (see Fig. 21.19). Animal products are dietary sources. Creatine is reversibly phosphorylated to creatine phosphate by creatine kinase, using ATP as the phosphate donor. [Note: The presence of creatine kinase (MB isozyme) in the plasma is indicative of heart damage and is used in the diagnosis of myocardial infarction (see p. 65).] 2.
Degradation: Creatine and creatine phosphate spontaneously cyclize at a slow but constant rate to form creatinine, which is excreted in the urine. The amount excreted is proportional to the total creatine phosphate content of the body and, therefore, can be used to estimate muscle mass. When muscle mass decreases for any reason (for example, from paralysis or muscular dystrophy), the creatinine content of the urine falls. In addition, a rise in blood creatinine is a sensitive indicator of kidney malfunction, because creatinine normally is rapidly cleared from the blood and excreted. A typical adult male excretes ~1–2 g of creatinine/day.
E. Melanin
Melanin is a pigment that occurs in several tissues, particularly the eye, hair, and skin. It is synthesized from tyrosine in melanocytes (pigmentforming cells) of the epidermis. It functions to protect underlying cells from the harmful effects of sunlight. [Note: A defect in melanin production results in oculocutaneous albinism, the most common type being due to defects in copper-containing tyrosinase (see p. 273).]
IV. CHAPTER SUMMARY