Biochemistry: Enzymes, vitamins and trace elements

🧪 Enzymes, vitamins and trace elements. Enzymes speed up every reaction in the body, and their kinetics explain drug actions, poisoning and laboratory measurements. Vitamins and trace elements are the cofactors many enzymes need; deficiency or excess produces classic syndromes that still appear in UK practice - in older people, people with alcohol dependence or malabsorption, after bariatric surgery, and in eating disorders. This chapter covers enzyme kinetics, enzymes as diagnostic markers, and each vitamin and trace element with its deficiency and toxicity states.
Your four questions for a suspected deficiency: Is intake, absorption or utilisation the problem? Which tissues are affected first? Which test confirms it (and is it reliable in acute illness)? How quickly should it be replaced, and by which route?

---

Enzyme Kinetics

• Enzymes lower the activation energy of reactions; they are not consumed and do not change the equilibrium.
• Michaelis–Menten kinetics: reaction velocity rises with substrate concentration until enzymes are saturated (Vmax). Km = substrate concentration at half Vmax - a low Km means high affinity.
• First-order kinetics - a constant proportion of drug is eliminated per unit time (most drugs at therapeutic doses; half-life is constant).
• Zero-order (saturation) kinetics - a constant amount is eliminated per unit time because the enzyme is saturated: ethanol, phenytoin, high-dose aspirin (salicylate) - small dose increases cause large level rises; half-life lengthens with dose.
• Cofactors - metal ions (Zn²⁺, Mg²⁺, Fe²⁺, Cu²⁺) or organic coenzymes (mostly derived from B vitamins).
• Isoenzymes - different forms catalysing the same reaction (e.g. CK-MB in heart, LDH isoforms) - tissue-specific diagnostic use.

Enzyme Inhibition and Drug Action

Type	Effect on Km	Effect on Vmax	Example
Competitive	↑ (overcome by more substrate)	Unchanged	Methotrexate (DHFR), statins (HMG-CoA reductase), fomepizole and ethanol (alcohol dehydrogenase in methanol/ethylene glycol poisoning)
Non-competitive	Unchanged	↓	Some metal ions; allosteric inhibitors
Irreversible	-	↓ (until new enzyme made)	Aspirin (COX - platelets lack nuclei, so the effect lasts the platelet lifespan of 7–10 days), organophosphates (acetylcholinesterase), PPIs (H⁺/K⁺-ATPase), MAOIs (some)

• Cytochrome P450 enzymes (mainly CYP3A4, 2D6, 2C9, 2C19, 1A2) metabolise most drugs. Inducers (rifampicin, carbamazepine, phenytoin, St John's wort, chronic alcohol, smoking for 1A2) reduce drug levels over days to weeks; inhibitors (clarithromycin, erythromycin, azole antifungals, ritonavir, grapefruit juice, amiodarone, fluoxetine, cimetidine, valproate, isoniazid) increase levels quickly.
• Pharmacogenetics: CYP2D6 ultra-rapid metabolisers convert codeine to morphine excessively (toxicity - avoid codeine in breastfeeding and under-12s); CYP2C19 poor metabolisers activate clopidogrel poorly; TPMT deficiency and azathioprine toxicity; DPD deficiency and 5-FU/capecitabine toxicity; HLA-B*57:01 and abacavir hypersensitivity.

Enzymes as Diagnostic Markers

Enzyme	Source	Raised in
ALT	Liver (cytoplasm) - specific	Hepatocellular injury (viral hepatitis, NAFLD, drugs, ischaemia)
AST	Liver, heart, muscle, red cells	Hepatocellular injury; AST:ALT >2 suggests alcohol-related liver disease
ALP	Liver (biliary), bone, placenta, intestine	Cholestasis (with ↑GGT), bone disease (Paget's, metastases, osteomalacia, fractures, growth), pregnancy
GGT	Liver (biliary), induced by alcohol and drugs	Cholestasis, alcohol, enzyme-inducing drugs; a raised GGT confirms that a raised ALP is hepatic
CK	Skeletal and cardiac muscle	Rhabdomyolysis, myositis, statin myopathy, IM injections, exercise, hypothyroidism, muscular dystrophy
LDH	All tissues	Haemolysis, lymphoma (tumour marker), tissue necrosis, PCP
Amylase / lipase	Pancreas (amylase also salivary glands)	Acute pancreatitis (>3× upper limit); amylase also raised in perforation, ischaemic bowel, DKA, renal failure, macroamylasaemia
Troponin (protein, not an enzyme)	Cardiac myocytes	Myocardial injury - MI, myocarditis, PE, sepsis, renal failure, tachyarrhythmia

• Pattern recognition: hepatitic (ALT ≫ ALP) versus cholestatic (ALP and GGT ≫ ALT) liver tests; isolated raised ALP with normal GGT suggests bone; isolated raised bilirubin (unconjugated) with normal enzymes in a well person suggests Gilbert's syndrome.

Fat-soluble Vitamins - A, D, E, K

• Absorbed with fat - deficiency in malabsorption (coeliac disease, cystic fibrosis, cholestasis, pancreatic insufficiency, orlistat, bariatric surgery, short bowel); stored in the body, so toxicity is possible (A and D).

Vitamin	Function	Deficiency	Toxicity
A (retinol)	Vision (rhodopsin), epithelial integrity, immunity	Night blindness, xerophthalmia, Bitot's spots, keratomalacia (leading preventable cause of childhood blindness worldwide), infections (measles severity)	Teratogenic (isotretinoin, high-dose supplements, liver), raised intracranial pressure, hepatotoxicity, hypercalcaemia
D (calciferol)	Calcium and phosphate absorption, bone mineralisation	Rickets (children - bowed legs, rachitic rosary, widened wrists), osteomalacia (adults - bone pain, proximal myopathy, Looser's zones), hypocalcaemia	Hypercalcaemia (excess supplements, sarcoidosis and lymphoma via extra-renal 1α-hydroxylase)
E (tocopherol)	Antioxidant in membranes	Haemolytic anaemia (preterm infants), spinocerebellar degeneration, peripheral neuropathy, retinopathy (fat malabsorption, abetalipoproteinaemia)	Bleeding tendency at high doses (antagonises vitamin K)
K (phylloquinone)	γ-carboxylation of clotting factors II, VII, IX, X and proteins C and S	Bleeding, raised PT/INR; vitamin K deficiency bleeding of the newborn (give vitamin K at birth)	Rare (synthetic menadione → haemolysis in infants)

• Vitamin D status - measure 25-hydroxyvitamin D: <25 nmol/L deficient, 25–50 insufficient (UK). Treatment: loading (e.g. colecalciferol 300,000 IU over 6–10 weeks), then maintenance 800–2,000 IU daily; check calcium after loading.
• Warfarin blocks vitamin K epoxide reductase - reversed with vitamin K (slowly) and prothrombin complex concentrate (fast).

The B Vitamins

Vitamin	Coenzyme role	Deficiency
B1 - thiamine	PDH, α-ketoglutarate dehydrogenase, transketolase	Wernicke's encephalopathy (confusion, ataxia, ophthalmoplegia/nystagmus - only a minority have all three) → Korsakoff's syndrome (amnesia with confabulation); dry beriberi (neuropathy), wet beriberi (high-output heart failure). Give IV Pabrinex before glucose in at-risk people
B2 - riboflavin	FAD, FMN	Angular cheilitis, glossitis, corneal vascularisation
B3 - niacin	NAD, NADP	Pellagra - the 3 Ds: dermatitis (photosensitive, Casal's necklace), diarrhoea, dementia (and death); causes - carcinoid syndrome (tryptophan diverted to serotonin), isoniazid, Hartnup disease, alcohol, maize diets. Niacin excess - flushing, hepatotoxicity
B5 - pantothenic acid	Coenzyme A	Rare - burning feet
B6 - pyridoxine	Transamination, haem synthesis, neurotransmitter synthesis	Peripheral neuropathy (isoniazid - give pyridoxine prophylaxis), sideroblastic anaemia, seizures in infants, glossitis. Excess → sensory neuropathy
B7 - biotin	Carboxylases (pyruvate carboxylase, acetyl-CoA carboxylase)	Rare - raw egg white (avidin), dermatitis, alopecia. High-dose supplements interfere with immunoassays (falsely abnormal thyroid tests, troponin)
B9 - folate	One-carbon transfer, DNA synthesis	Megaloblastic anaemia, neural tube defects (see below)
B12 - cobalamin	Methionine synthase, methylmalonyl-CoA mutase	Megaloblastic anaemia, neuropathy (see below)

Vitamin B12 and Folate

• Both are needed for DNA synthesis - deficiency causes megaloblastic anaemia (macrocytosis, hypersegmented neutrophils, pancytopenia, raised LDH and bilirubin from ineffective erythropoiesis).
• B12 absorption: released from food by acid and pepsin → binds haptocorrin → released in the duodenum by pancreatic proteases → binds intrinsic factor (gastric parietal cells) → absorbed in the terminal ileum. Body stores last 2–4 years.
• Causes of B12 deficiency: pernicious anaemia (autoimmune - intrinsic factor and parietal cell antibodies; associated with other autoimmune diseases and gastric cancer), gastrectomy and bariatric surgery, terminal ileal disease or resection (Crohn's), metformin, PPIs (long-term), nitrous oxide misuse (inactivates B12), vegan diet, fish tapeworm, bacterial overgrowth.
• Neurological effects of B12 deficiency - subacute combined degeneration of the cord (dorsal columns - loss of vibration and joint position sense, sensory ataxia, positive Romberg; corticospinal tracts - spastic weakness, extensor plantars with absent ankle jerks), peripheral neuropathy, cognitive impairment, optic atrophy. Can occur without anaemia.
• Treatment: IM hydroxocobalamin (see the Hydroxocobalamin topic); never give folic acid alone to someone with B12 deficiency - it may correct the anaemia while the neurological damage progresses.
• Folate: absorbed in the jejunum; stores last about 4 months. Deficiency - poor diet, alcohol, malabsorption (coeliac), increased demand (pregnancy, haemolysis, malignancy, psoriasis), drugs (methotrexate, trimethoprim, phenytoin, sulfasalazine). Folate deficiency raises homocysteine; B12 deficiency raises both homocysteine and methylmalonic acid.

Vitamin C

• Functions: hydroxylation of proline and lysine in collagen synthesis, antioxidant, enhances non-haem iron absorption.
• Scurvy - still seen in older people living alone, alcohol dependence, restrictive diets and some children with autism or food selectivity: perifollicular haemorrhages and corkscrew hairs, swollen bleeding gums, poor wound healing, bruising, bone pain, anaemia, fatigue.
• Excess - oxalate kidney stones; can interfere with glucose meters.

Iron Metabolism

• Absorption in the duodenum (as Fe²⁺ via DMT1) - enhanced by acid and vitamin C; reduced by PPIs, tea, calcium, phytates. Exported by ferroportin; transported by transferrin; stored as ferritin and haemosiderin.
• Hepcidin (from the liver) blocks ferroportin → reduces absorption and traps iron in macrophages. Hepcidin rises with inflammation (anaemia of chronic disease - low serum iron, normal or high ferritin) and falls with iron deficiency and hypoxia.
• No physiological excretion pathway - iron balance is controlled at absorption; losses are from bleeding and shed cells.

Test	Iron deficiency	Anaemia of chronic disease	Haemochromatosis
Ferritin	↓ (best single test)	Normal or ↑ (acute phase reactant)	↑↑
Transferrin saturation	↓	↓ or normal	↑ (>45–50%)
TIBC / transferrin	↑	↓	↓ or normal

• Hereditary haemochromatosis - HFE C282Y homozygosity (northern European ancestry); low hepcidin → iron overload: liver cirrhosis and hepatocellular carcinoma, diabetes, skin pigmentation ("bronze diabetes"), arthropathy (2nd and 3rd MCP joints), cardiomyopathy, hypogonadism; treat with venesection.
• Iron overdose - GI toxicity, then shock, metabolic acidosis, liver failure; desferrioxamine.

Zinc, Copper, Selenium, Iodine and Others

Element	Role	Deficiency	Excess
Zinc	Enzyme cofactor, wound healing, immunity, taste	Acrodermatitis (periorificial dermatitis), alopecia, diarrhoea, poor wound healing, loss of taste; acrodermatitis enteropathica (inherited); after bariatric surgery or with parenteral nutrition	Copper deficiency (zinc blocks copper absorption)
Copper	Cytochrome oxidase, lysyl oxidase (collagen), ceruloplasmin	Anaemia, neutropenia, myeloneuropathy mimicking B12 deficiency (after bariatric surgery, excess zinc); Menkes disease (X-linked - kinky hair)	Wilson's disease (ATP7B - liver disease, neuropsychiatric features, Kayser–Fleischer rings, low ceruloplasmin, high urinary copper)
Selenium	Glutathione peroxidase, deiodinases	Cardiomyopathy (Keshan disease), myopathy	Hair and nail loss, garlic breath
Iodine	Thyroid hormone synthesis	Goitre, hypothyroidism, cretinism (iodine deficiency in pregnancy - leading preventable cause of intellectual disability worldwide)	Hypo- or hyperthyroidism
Fluoride	Tooth enamel	Dental caries	Fluorosis (mottled teeth), skeletal fluorosis
Chromium, manganese, molybdenum	Minor cofactors	Rare - long-term parenteral nutrition	Manganese - parkinsonism

Groups at Risk of Deficiency

• Alcohol dependence - thiamine, folate, B6, niacin, magnesium, zinc, vitamin A.
• Older people living alone - vitamin D, vitamin C, folate, B12, protein-energy malnutrition.
• Bariatric surgery - iron, B12, folate, calcium, vitamin D, thiamine (vomiting), copper, zinc; lifelong supplementation and monitoring.
• Malabsorption (coeliac, Crohn's, CF, short bowel) - fat-soluble vitamins, iron, folate, B12 (ileal).
• Vegan diets - B12 (supplement), iodine, iron, calcium, vitamin D, omega-3.
• Eating disorders and refeeding - thiamine, phosphate, potassium, magnesium.
• Pregnancy - folate, iron, vitamin D, iodine.
• Darker skin, covered skin, housebound - vitamin D.

Vitamins at a Glance

Vitamin	Classic deficiency
A	Night blindness, xerophthalmia
D	Rickets, osteomalacia
E	Haemolysis, neuropathy, ataxia
K	Bleeding (II, VII, IX, X)
B1	Wernicke–Korsakoff, beriberi
B3	Pellagra (dermatitis, diarrhoea, dementia)
B6	Neuropathy (isoniazid), sideroblastic anaemia
B12	Megaloblastic anaemia, SACD
Folate	Megaloblastic anaemia, neural tube defects
C	Scurvy

Spot the Diagnosis

• Confused, ataxic man with nystagmus who drinks heavily. → Wernicke's encephalopathy - IV Pabrinex now.
• Photosensitive rash, diarrhoea and confusion in a patient with metastatic carcinoid. → Pellagra (tryptophan diverted to serotonin).
• Bleeding gums and corkscrew hairs in a man who eats only tea and toast. → Scurvy.
• Gait ataxia, absent ankle jerks and extensor plantars with a raised MCV. → Subacute combined degeneration (B12 deficiency).
• Numbness and ataxia in a young person using nitrous oxide recreationally. → Functional B12 deficiency (myeloneuropathy).
• Pancytopenia and myeloneuropathy after gastric bypass, normal B12. → Copper deficiency.

Exam-style Scenarios

• Scenario. A patient on phenytoin develops toxicity after a small dose increase from 300 to 350 mg daily. Why?
  Answer: Phenytoin follows zero-order (saturation) kinetics at therapeutic levels - the metabolising enzyme is saturated, so small dose increases cause disproportionate rises in plasma levels.
• Scenario. A 60-year-old with a raised ALP has a normal GGT and bone pain in the pelvis; calcium is normal. What is likely?
  Answer: A bone source of ALP - Paget's disease of bone is likely (normal calcium and phosphate, very high ALP); X-ray and bone scan; bisphosphonate if symptomatic.
• Scenario. A woman with coeliac disease has night blindness and easy bruising. Which deficiencies are present?
  Answer: Fat-soluble vitamin malabsorption - vitamin A (night blindness) and vitamin K (bruising, prolonged PT).

Key Points

• Zero-order kinetics - ethanol, phenytoin, salicylate overdose.
• Know the CYP inducers and inhibitors and key pharmacogenetic tests (TPMT, DPD, HLA-B*57:01).
• A raised ALP with normal GGT points to bone; ferritin is the best single test for iron deficiency (but is raised by inflammation).
• Thiamine before glucose; never folate alone in B12 deficiency.
• Fat-soluble vitamin deficiency signals malabsorption.

Guidelines and Further Reading

• National Osteoporosis Society / Royal Osteoporosis Society: Vitamin D and bone health - a practical clinical guideline
• British Society for Haematology: Diagnosis and treatment of cobalamin and folate disorders; NICE NG239: Vitamin B12 deficiency in over 16s
• British Society of Gastroenterology: Guidelines for the management of iron deficiency anaemia in adults
• NICE CG100: Alcohol-use disorders - physical complications (thiamine)
• British Obesity and Metabolic Surgery Society: Guidelines on perioperative and postoperative biochemical monitoring and micronutrient replacement