Browsing Pathways

Methylhistidine is a modified amino acid that is produced in myocytes during the methylation of actin and myosin. It is also formed from the methylation of L-histidine, which takes the methyl group from S-adenosylmethionine and forms S-adenosylhomocysteine as a byproduct. After its formation in the myocytes, methylhistidine enters the blood stream and travels to the kidneys, where it is excreted in the urine. Methylhistidine is present in the blood and urine in higher concentrations after skeletal muscle protein breakdown, which can occur due to disease or injury. Because of this, it can be used to judge how much muscle breakdown is occurring. Methylhistidine levels are also affected by diet, and may differ between vegetarian diets and those containing meats.

Thyroid hormone synthesis is a process that occurs in the thyroid gland in humans that results in the production of thyroid hormones which regulate many different processes in the body, such as metabolism, temperature regulation and growth/development. Thyroid hormone synthesis begins in the nucleus of a thyroid follicular cell, as thyroglobulin synthesis occurs here and is transported to the endoplasmic reticulum. From there, thyroglobulin transported through endocytosis into the intracellular space, and then transported through exocytosis to the follicle colloid. There, thyroglobulin is joined by iodide that has been transported from the blood, through the thyroid follicular cell and arrived in the the follicle colloid using pendrin, and hydrogen peroxide to be catalyzed by thyroid peroxidase, creating thyroglobulin + iodotyrosine. Then, iodide, hydrogen peroxide and thyroidperoxidase create thyroglobulin + 3,5-diiodo-L-tyrosine. Thyroglobulin+3,5-diiodo-L-tyrosine then joins with hydrogen peroxide and thyroid peroxidase to create thyroglobulin + 2-aminoacrylic acid and thyroglobulin+liothyronine. Thyroglobulin + liothyronine then goes through two processes, the first being its transportation into the cell and undergoing of proteolysis, which is followed by liothyronine being transported into the bloodstream. The second process is thyroglobulin + liothyronine being catalyzed by thyroid peroxidase and resulting in the production of thyroglobulin + thyroxine. Thyroglobulin + thyroxine is then transported back into the cell, undergoes proteolysis, and thyroxine alone is transported back out of the cell and into the bloodstream.

Apparent mineralocorticoid excess (AME), also known as cortisol 11-beta-ketoreductase deficiency, is an extremely rare inborn error of metabolism (IEM) and autosomal recessive disorder of the steroidogenesis pathway. It is caused by a mutation in the HSD11B2 gene which encodes for corticosteroid 11-beta-dehydrogenase isozyme 2, and enzyme that converts cortisol to cortisone in the cell. Without this enzyme being functional, an accumulation of tetrahydrocortisol builds up, while tetrahydrocortisone levels dissipate. AME is characterized excessive thirst and urination, and along with this, symptoms include low levels of aldosterone, failure to thrive and hypertension. Treatment with corticoids that suppress the secretion of cortisol within the body can affect blood pressure and aldosterone levels. Antihypertensive agents are also effective. It is estimated that AME affects less than 1 in 1,000,000 individuals, with less than 100 reported cases as of 2019.

3-beta-hydroxysteroid dehydrogenase (HSD) deficiency is an extremely rare inborn error of metabolism (IEM) and autosomal recessive disorder of the steroidogenesis pathway. It is caused by an defect in the HSD3B2 gene which encodes for the 3 beta-hydroxysteroid dehydrogenase enzyme, which is responsible for forming cortisol from 11b,17a,21-trihydroxypregnenolone. When the enzyme is not correctly produced, cortisol levels in the cell are lowered, and as cortisol is used in the production of other steroids, it may affect their levels as well. 3-beta-HSD deficiency is characterized by low levels of cortisol produced in the adrenal glands. Symptoms include abnormal genitalia for both males and females, as well as infertility. There is also a more severe salt-wasting form of this deficiency, characterized by dehydration. Treatment for 3-beta-HSD deficiency includes steroid replacement, as well as sex hormone replacement during puberty to allow proper development. Surgery can also be used to correct any genital abnormalities that may occur. It is estimated that 3-beta-HSD deficiency affects less than 1 in 1,000,000 individuals, with around 60 cases reported.

2-Aminoadipic 2-oxoadipic aciduria is a disorder of lysine metabolism caused by a defective DHTKD1 gene. DHTKD1 is predicted to code for a component of a supercomplex similar to the 2-oxoglutarate dehydrogenase complex (OGDHc) which catalyzes the conversion of 2-oxoadipate into glutaryl-CoA. This disease is characterized by a large accumulation of 2-oxoadipate and 2-hydroxyadipate in the urine. Symptoms of the disease include mild to severe intellectual disability, developmental delay, ataxia, muscular hypotonia, and epilepsy. However, most cases are asymptomatic.

Sterol 27-hydroxylase is a mitochondrial cytochrome P-450 species (CYP27) that catalyzes the first step in the degradation of steroid side chain in cholesterol to produce bile acids in the liver. When there are low concentrations of 27-Hydroxylase, patients will exhibit cerebrotendinous xanthomatosis, an autosomal recessive disorder characterized by the accumulation of cholestanol and cholesterol due to the inability to break down the lipids. The formation of xanthomas (deposits of lipids) in the nervous system and tendons will cause symptoms such as dementia, ataxia, and cataracts. Other symptoms may include damaged liver cells and body tissues.

3-Phosphoglycerate dehydrogenase deficiency is a disorder of L-serine biosynthesis that is characterized by congenital microcephaly, psychomotor retardation, and seizures.The disorder is caused by homozygous or compound heterozygous or homozygous mutation in the gene encoding phosphoglycerate dehydrogenase on chromosome 1p12. Defects in the gene lead to a decrease of Glycine and Serine.

Ocular non-nephropathic cystinosis, also known as adult-onset cystinosis, is a rare inborn error of metabolism (IEM) and autosomal recessive disorder of the cysteine metabolism pathway. It is caused by a defect in the CTNS gene, which encodes the protein cystinosin, which acts as a cystine/H+ symporter that transports L-cysteine out of the lysosome. Ocular non-nephropathic cystinosis is characterized by a buildup of cysteine in cells, in the case of this form in the cornea. Symptoms include photophobia and damage to the cornea due to crystals forming from the excess cysteine. However, unlike other forms of cystinosis, no or minimal kidney damage occurs. Treatment with cysteamine, a drug that can convert cysteine into a form that can be secreted by the lysosome, can be effective in all of the forms of cystinosis. It is estimated that ocular non-nephropathic cystinosis affects less than 1 in 100,000 to 200,000 individuals, which is the rate of the more severe nephropathic cystinosis.

Cystinuria is a genetic condition caused by mutations in the SLC7A9 or SLC3A1 gene. These two genes are responsible for creating subunits of a protein that reabsorbs cystine into the blood, located in the kidneys. The mutations cause this process to be compromised and allows the amino acids to build up and have a high concentration in urine. This causes crystals to form and become stones as they grow larger. These stones can become lodged in the bladder or in the kidneys which can cause pain, develop infection and disrupt the passing of urine through the urinary tract if the stones create a blockage there.

Hereditary folate malabsorption, also known as folic acid transport defect, is an extremely rare inborn error of metabolism (IEM) and autosomal recessive disorder of the folate metabolism pathway. It is caused by a defect in the SLC46A1 gene that encodes the proton-coupled folate transporter protein which is responsible for folate uptake from the intestines. Hereditary folate malabsorption is characterized by low concentrations of folate in the serum and cerebrospinal fluid. Symptoms include feeding difficulties and failure to thrive and anemia, as well as potential neurological issues such as seizures and developmental delays. When infants are born with hereditary folate malabsorption, there are initially few signs, as folate is provided across the placenta, but after birth, folate absorption is inhibited and these symptoms begin to be exhibited. Treatment for hereditary folate malabsorption includes intramuscular or oral doses of reduced folates to bring cerebrospinal fluid folate levels to a normal range, as well as blood transfusions if severe anemia is present. It is estimated that hereditary folate malabsorption affects less than 1 in 1,0000,000 people, with only approximately 30 reported cases.