Porphyrin metabolism & porphyrias

What are porphyrins ? Porphyrins Porphyrins are cyclic compounds that bind metal ions (usually Fe2+ or Fe3+) Metaloporphyrin Porphyrin + Metal = Metaloporphyrin heme Most prevalent metalloporphyrin in humans is heme (metal here is iron ion) Heme consists of Heme consists of: One ferrous ion One ferrous ion (Fe2+) in the centre Protoporphyrin IX Protoporphyrin IX (a tetrapyrrole ring) Heme is the prosthetic group Heme is the prosthetic group of hemoglobin, myoglobin, cytochromes, catalase, perioxidases, NOS and tryptophan pyrrolase So, heme + globin protein = hemoglobin

Structure of porphyrins Porphyrins Porphyrins are cyclic molecules formed by 4 pyrrole (tetrapyrrole) rings linked by methenyl bridges. Different porphyrins vary in the nature of side chains that are attached to each of the 4 pyrrole rings. Protoporphyrin IX contains vinyl, methyl & propionate Distribution of side chains type III porphyrins - Side chains can be ordered around tetrapyrrole nucleus in 4 different ways designated I, II, III & IV series. Only type III porphyrins are physiologically important in humans Protoporphyrin IX - Protoporphyrin IX is a member of type III series Porphyrinogens are porphyrin precursors intermediate between porphobilinogen & protoporphyrin Porphobilinogen Porphyrinogens Protoporphyrins (A), Pyrrole ring; (B), porphin ring; (C), protoporphyrin IX.

Heme- Fe 2+ (ferrous) protoporphyrin Hemin- Fe 3+ (ferric) protoporphyrin

Biosynthesis of Heme Heme synthesis occurs in all cells except RBCs due to the requirement for heme as a prosthetic group on enzymes and electron transport chain. By weight, the major locations of heme synthesis are the liver and the erythroid progenitor cells of the bone marrow. Key enz

Clinical importance of first step (ALA synthase control): # When heme (end product) is produced in excessive amounts, heme is converted hemin to hemin. Hemin decreases action of ALA synthase in liver. (end product inhibition).The reverse occurs when heme biosynthesis is reduced. # Drugs # Drugs as grisofulvin (antifungal), hydantoin & phenobarbital (anticonvulsant) cytochrome p450 increase ALA synthase activity: as these drugs are metabolized by cytochrome p450 in liver resulting in more consumption of heme (component of cytochrome). Accordingly, heme concentration is reduced resulting in stimulation of action of ALA synthase.

Cytochrome P450 Monooxygenase System: heme- containingmonooxygenase enzymes Cytochrome P450s (CYPs) are actually a superfamily of related, heme- containing monooxygenase enzymes that participate in abroad variety of reactions. This system performs different functions in two separate locations in cells. The over-all reaction catalyzed by a cytochrome P450 enzyme is: RR R-H + O2+ NADPH + H+ → R-OH + H2O NADP+ R where R may be a steroid, drug, or other chemical. The name P450 reflects the absorbance at 450 nm by the protein. Role of cytochrome P450 in detoxification of drugs & toxic compounds: drug It may itself activate or inactivate a drug toxic compound It can make a toxic compound more soluble, thus facilitating its excretion in the urine or feces.

Step 2: Formation of porphobilinogen Step 2: Formation of porphobilinogen: porphobilinogen ALA dehydratase 2 molecules of  -Amino levulinic acid (ALA) condense to form porphobilinogen by the enzyme ALA dehydratase. Clinical importance Clinical importance: lead ALA dehydratase enzyme is inhibited by heavy metals as lead that results in anemia. (lead poisoning). In this case: ALA in blood is elevated (lab investigation) Biosynthesis of heme Biosynthesis of heme (cont.)

Further steps: (in mitochondria) protoporphyrin IX Formation of protoporphyrin IX ferrous ions (Fe2+) are introduced into protoporphyrin IX Then, ferrous ions (Fe2+) are introduced into protoporphyrin IX, either: simultaneously ferrochelatase or: enhanced by ferrochelatase Clinical importance Clinical importance: Ferrochelatase enzyme is inhibited by lead

Porphyrias Porphyria Porphyria are rare inherited defects in heme synthesis. An inherited defect in an enzyme of heme synthesis results in accumulation of one or more of porphyrins and their precursors depending on location of block of the heme synthesis pathway. These porphyrins & precursors increase in blood & appear in urine of patients. Porphyria Porphyria means purple colour caused by pigment-like porphyrins in urine of patients. (Diagnosed by lab investigation) autosomal dominant pattern Most porphyrias show a prevalent autosomal dominant pattern, except congenital eythropoietic porphyria, which is recessive

Clinical manifestations of porphyrias Clinical manifestations of porphyrias: Two types of porphyrias Two types of porphyrias: erythropoietic (bone marrow) & hepatic Hepatic porphyrias are Hepatic porphyrias are: acute & chronic porphyrias A- Neurological Manifestations: prior tetrapyrroles Generally, individuals with an enzyme defect prior to the synthesis of the tetrapyrroles manifest abdominal and neuropsychiatric signs B- Cutaneous Manifestations: with tetrapyrrole intermediates Those with tetrapyrrole intermediates show photosensitivity with formation of reactive oxygen species (ROS) that damage membranes by oxidation resulting in the following effects: - Skin blisters, itches (pruritis) - Skin may darken, grow hair (hypertrichosis)

Porphyria Cutanea Tarda Chronic hepatic porphyria The most common type of porphyria uroporphyrinogen decarboxylase a deficiency in uroporphyrinogen decarboxylase Clinical expression of the enzyme deficiency is influenced by various factors, such as exposure to sunlight, the presence of hepatitis B or C Clinical onset is during the fourth or fifth decade of life. cutaneous symptoms urine Porphyrin accumulation leads to cutaneous symptoms and urine that is red to brown in natural light and pink to red in fluorescent light Acute Hepatic Porphyrias e.g. Acute Intermittent Porphyria Porphyrias leading to accumulation of ALA and porphobilinogen cause abdominal pain and neuropsychiatric disturbances, ranging from anxiety to delirium. Symptoms of the acute hepatic porphyrias are often precipitated by administration of drugs such as barbiturates and ethanol.

Types of Porphyrias

RBCs last 120 days then, degraded by Reticulo-endothelial System (RES) (in liver & spleen) Hemoglobin (released from RBCs) Heme + globin (reused) Biliverdin Bilirubin Liver Bile Intestine feces Degradation of Heme