Which enzymes require biotin?

Biotin is the cofactor for four carboxylases: pyruvate carboxylase (gluconeogenesis), acetyl-CoA carboxylase (the rate-limiting step of fatty acid synthesis), propionyl-CoA carboxylase (breakdown of valine, isoleucine, methionine, threonine and odd-chain fatty acids) and 3-methylcrotonyl-CoA carboxylase (leucine breakdown). Each reaction uses ATP and bicarbonate to add a carboxyl group, with biotin carrying the CO2.

Why do raw egg whites cause biotin deficiency?

Raw egg white contains avidin, a glycoprotein that binds biotin tightly and resists digestion, so dietary biotin cannot be absorbed. Deficiency develops only after regular raw egg white intake for many weeks to years. Cooking denatures avidin, so cooked eggs do not cause deficiency, and the yolk itself supplies biotin.

What are the symptoms of biotin deficiency?

Classic findings are thinning hair that can progress to loss of all body hair, a scaly red rash around the eyes, nose, mouth and perineum, conjunctivitis and brittle nails; board reviews summarize it as dermatitis, alopecia and enteritis. Adults can develop depression, lethargy, hallucinations and paresthesias, while infants develop hypotonia, lethargy and developmental delay. Severe cases can have seizures and ketolactic acidosis.

What is biotinidase deficiency?

Biotinidase deficiency is an autosomal recessive disorder in which the body cannot release biotin from dietary protein or recycle it, so all biotin-dependent carboxylases underperform (multiple carboxylase deficiency). Untreated profound deficiency causes seizures, hypotonia, rash, alopecia, ataxia, developmental delay and hearing and vision loss, usually beginning in the first months of life. It is picked up by newborn screening and treated with lifelong oral biotin, and children treated early usually develop normally.

Can biotin supplements affect lab test results?

Yes. Many immunoassays rely on biotin-streptavidin binding, and high-dose biotin, such as 5 to 10 mg hair, skin and nail supplements, can make sandwich assays like TSH and troponin falsely low and competitive assays like free T4 falsely high. A falsely low troponin can hide a heart attack, and the FDA has reported a death after one such result. The American Thyroid Association advises stopping biotin at least 2 days before thyroid testing.

Does biotin deficiency raise methylmalonic acid?

Methylmalonic acid is the marker of vitamin B12 deficiency, not biotin deficiency. Propionyl-CoA carboxylase, the biotin-dependent enzyme, acts upstream of methylmalonyl-CoA mutase, the B12-dependent enzyme. The typical urinary marker of biotin deficiency is 3-hydroxyisovaleric acid, which reflects slowed leucine breakdown.

Biotin (Vitamin B7): 4 Carboxylases, Deficiency & Avidin

Biotin (vitamin B7) is the CO₂ carrier for four ATP-dependent carboxylases: pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase and 3-methylcrotonyl-CoA carboxylase. Deficiency is rare but a board classic: raw egg whites (avidin binds biotin), long-term antibiotics or anticonvulsants, parenteral nutrition without biotin and inherited biotinidase deficiency cause dermatitis around the body openings, alopecia, conjunctivitis, enteritis and neurologic signs, sometimes with ketolactic acidosis. The newer high-yield twist: high-dose biotin supplements distort streptavidin-based immunoassays, giving falsely low TSH and troponin and falsely high free T4; the FDA has warned about biotin interference since 2017.

The four biotin-dependent carboxylases

Every biotin enzyme is a carboxylase. Holocarboxylase synthetase links biotin to a lysine residue on each enzyme, and each reaction uses ATP + bicarbonate to add a carboxyl group to its substrate. Memory hook: carboxylases run on ABC: ATP, Biotin, CO₂.

Enzyme	Reaction	Where	Pathway and exam angle
Pyruvate carboxylase	Pyruvate → oxaloacetate	Mitochondria	First step of gluconeogenesis; refills the TCA cycle (anaplerosis); allosterically activated by acetyl-CoA
Acetyl-CoA carboxylase (ACC1, ACC2)	Acetyl-CoA → malonyl-CoA	Cytosol (ACC1); outer mitochondrial membrane (ACC2)	Rate-limiting step of fatty acid synthesis; malonyl-CoA also inhibits CPT-1, so new fatty acids aren't burned
Propionyl-CoA carboxylase	Propionyl-CoA → methylmalonyl-CoA	Mitochondria	Breaks down propionyl-CoA from valine, odd-chain fatty acids, methionine, isoleucine and threonine (VOMIT) and the cholesterol side chain; downstream, methylmalonyl-CoA mutase needs B12
3-Methylcrotonyl-CoA carboxylase	3-Methylcrotonyl-CoA → 3-methylglutaconyl-CoA	Mitochondria	Leucine breakdown; when it slows, urinary 3-hydroxyisovaleric acid rises, a marker of biotin deficiency

The B-vitamin cofactor map, including thiamine's decarboxylases, is drilled in the enzyme cofactors mnemonic, and ACC regulation by insulin and citrate is covered in fatty acid synthesis.

Absorption, recycling and the avidin trap

Food biotin is mostly protein-bound. Digestion leaves biocytin (biotin-lysine) and biotin peptides, and biotinidase frees the biotin, which is absorbed in the small intestine and stored mainly in the liver.
Two enzymes run the biotin cycle: holocarboxylase synthetase attaches biotin to the carboxylases, and biotinidase recycles it from biocytin when the enzymes are degraded. A defect in either one lowers all four carboxylases at once: multiple carboxylase deficiency.
Sources: liver, eggs, fish, meat, seeds, nuts and sweet potatoes. Adults need about 30 mcg/day (an adequate intake; no RDA has been set).
Avidin: a glycoprotein in raw egg white that binds biotin tightly and resists digestion, so biotin isn't absorbed. Cooking denatures avidin, so cooked eggs are safe. Deficiency takes regular raw egg white intake for many weeks to years.

Causes of biotin deficiency

Cause	Mechanism	Classic stem
Raw egg whites	Avidin blocks absorption	Bodybuilder on raw egg-white shakes for months
Long-term antibiotics	Board-classic association; may cut gut bacterial biotin synthesis, although how much that matters in humans is unknown	Rash and hair loss after prolonged antibiotics
Parenteral nutrition without biotin	No intake	Long-term TPN
Long-term anticonvulsants	Raise requirements and lower serum biotin	Years of antiseizure therapy
Chronic alcohol use	Inhibits absorption	Malnourished drinker
Pregnancy	Higher requirement; marginal deficiency is common	Usually subclinical
Biotinidase deficiency	Can't free or recycle biotin (BTD, autosomal recessive)	Infant a few months old with seizures, hypotonia, rash and alopecia
Holocarboxylase synthetase deficiency	Can't attach biotin to the carboxylases (HLCS, autosomal recessive)	Sick young infant with acidosis, rash and alopecia

What deficiency looks like

System	Findings
Skin	Scaly red rash around the eyes, nose, mouth and perineum; with unusual facial fat distribution this is called "biotin-deficient facies"; skin infections
Hair and nails	Thinning hair that can progress to loss of all body hair; brittle nails
Eyes	Conjunctivitis
Gut	Enteritis: the board triad is dermatitis, alopecia and enteritis
Nervous system, adults	Depression, lethargy, hallucinations, paresthesias of the extremities; ataxia and seizures
Nervous system, infants	Hypotonia, lethargy, developmental delay
Metabolic	Ketolactic acidosis and organic aciduria (↑ 3-hydroxyisovaleric acid)

Biotinidase vs holocarboxylase synthetase deficiency

Feature	Biotinidase deficiency	Holocarboxylase synthetase deficiency
Defect	Can't release biotin from food protein or recycle it from biocytin	Can't attach biotin to the carboxylases
Typical onset	Usually the first few months of life, sometimes later in childhood (also called late-onset multiple carboxylase deficiency)	Typically the first few months of life, but onset varies (also called early-onset or infantile multiple carboxylase deficiency)
Key features	Seizures, hypotonia, breathing problems, ataxia, developmental delay, skin rash, alopecia, candidiasis, hearing and vision loss	Feeding and breathing problems, rash, alopecia, lethargy; developmental delay, seizures and coma if untreated
Labs	Low serum biotinidase activity: profound below 10% of normal, partial 10–30%; ketolactic acidosis and organic aciduria	Normal biotinidase activity with a similar organic acid pattern
Treatment	Lifelong oral biotin in milligram doses (up to 5–20 mg/day is sometimes needed)	Lifelong high-dose biotin (10–80 mg/day); response varies

Biotinidase deficiency is part of US newborn screening and affects about 1 in 60,000 newborns.
Treated from the newborn period, children usually stay healthy with normal development. Once developmental delay, vision problems or hearing loss appear, the damage may be permanent.
Partial deficiency may cause hypotonia, rash and hair loss only during illness or stress.

Diagnosis and treatment

Acquired deficiency is usually suspected clinically: the rash and hair loss plus a risk factor. Supportive markers are low urinary biotin and high urinary 3-hydroxyisovaleric acid; the most reliable markers are reduced biotinylated 3-methylcrotonyl-CoA and propionyl-CoA carboxylases in white blood cells.
Infants: serum biotinidase activity (low in biotinidase deficiency, normal in holocarboxylase synthetase deficiency) plus urine organic acids.
Treatment: oral biotin and removing the cause (cook the eggs). Biotin is not known to be toxic.

Biotin and lab tests: the FDA warning

Many immunoassays capture biotin-labeled antibodies on streptavidin-coated surfaces. Excess biotin in the blood competes for streptavidin, so less of the test complex is captured:

Assay design	Examples	Effect of excess biotin
Sandwich (signal rises with analyte)	TSH, troponin, PTH, hCG	Falsely low
Competitive (signal falls as analyte rises)	Free T4, free T3	Falsely high

Thyroid pattern: low TSH with high T4 and T3 in a well patient looks like hyperthyroidism and has led to misdiagnosis.
Troponin: a falsely low result can hide a myocardial infarction. FDA safety communications in 2017 and 2019 warned about biotin interference, and in 2022 the FDA said it was still receiving adverse event reports of falsely low troponin results; the NIH Office of Dietary Supplements cites an FDA report of a patient on high-dose biotin who died after one such result.
Doses: hair, skin and nail supplements usually contain 5–10 mg, far above the 30 mcg adequate intake, and the FDA notes intakes as high as 300 mg/day. Biotin in ordinary multivitamins (up to 1 mg) has not been reported to interfere.
What to do: ask every patient about supplements. The American Thyroid Association advises stopping biotin at least 2 days before thyroid testing; laboratory medicine guidance suggests waiting at least 8 hours after a 5–10 mg dose and at least 72 hours after high-dose therapy (100 mg/day or more).

Mimics and exam traps

Trap	How to tell it apart
Vitamin B12 deficiency	Methylmalonyl-CoA mutase is downstream of propionyl-CoA carboxylase, so B12 deficiency raises methylmalonic acid (and homocysteine) with megaloblastic anemia and subacute combined degeneration; biotin deficiency acts upstream and also slows leucine breakdown, so its urinary marker is 3-hydroxyisovaleric acid, not methylmalonic acid
Zinc deficiency	Also periorificial and acral dermatitis, alopecia and diarrhea, but with impaired taste and smell, poor wound healing and hypogonadism; the inherited form, acrodermatitis enteropathica, starts after weaning
Thiamine and lipoic acid (lipoamide)	The pyruvate, α-ketoglutarate and branched-chain α-ketoacid dehydrogenases run oxidative decarboxylations with thiamine pyrophosphate (E1) and lipoamide (E2), which, like biotin, is attached to a lysine; thiamine also runs transketolase. None of these are carboxylases
Vitamin K	γ-Carboxylation of clotting factors uses vitamin K, not biotin
Egg question	Only raw egg white matters; the yolk is a good biotin source, and cooking destroys avidin

Practice question

A 24-year-old man preparing for a bodybuilding competition has blended several raw egg whites into his shakes every day for 6 months. He has thinning scalp hair, a scaly red rash around his eyes, nose and mouth, bilateral conjunctivitis, low mood and tingling in his feet. Which enzyme is most likely to have reduced activity?

A. Methylmalonyl-CoA mutase
B. Propionyl-CoA carboxylase
C. Pyruvate dehydrogenase
D. Lysyl oxidase
E. γ-Glutamyl carboxylase

Answer: B. Avidin in raw egg white binds dietary biotin, and all four biotin-dependent carboxylases lose activity, including propionyl-CoA carboxylase.

A: needs vitamin B12 (adenosylcobalamin); B12 deficiency raises methylmalonic acid and causes megaloblastic anemia.
C: an oxidative decarboxylation that uses thiamine and lipoic acid, not a biotin-dependent carboxylation.
D: needs copper, for collagen and elastin cross-linking.
E: a carboxylase, but vitamin K-dependent; it activates clotting factors.

Exam twist: a woman taking a 10 mg hair-and-nail supplement has a low TSH and high free T4 but no symptoms. The best next step is to stop biotin for at least 2 days and repeat the tests, not to start methimazole.

Key takeaways

Biotin = carboxylation (ABC: ATP, Biotin, CO₂): pyruvate, acetyl-CoA, propionyl-CoA and 3-methylcrotonyl-CoA carboxylases.
Causes: raw egg white avidin, long-term antibiotics or anticonvulsants, TPN without biotin, alcohol, pregnancy, and biotinidase or holocarboxylase synthetase deficiency.
Findings: periorificial dermatitis, alopecia, conjunctivitis, enteritis, depression, paresthesias and seizures; infants have hypotonia and developmental delay.
Biotinidase deficiency: autosomal recessive and newborn-screened; untreated, symptoms usually start in the first few months of life, and hearing and vision loss may be permanent, so treat early with lifelong biotin.
Methylmalonic acid points to B12, not biotin: propionyl-CoA carboxylase (biotin) acts upstream of methylmalonyl-CoA mutase (B12).
High-dose biotin interferes with immunoassays: falsely low TSH and troponin, falsely high free T4 and T3; stop it before testing.