Derivatives of Hemoglobin in animals

Derivatives of Hemoglobin in animals are Oxyhemoglobin, Myoglobin, Carboxyhemoglobin (HbCO) and Methemoglobin (ferrihemoglobin).

Oxyhemoglobin

Hemoglobin has an important physiological relationship with oxygen. Oxygen forms loose and reversible combination with hemoglobin called oxyhe­moglobin when the erythrocytes passes through the pulmonary capillaries. Since there are four ferrous atoms in the hemoglobin molecule, four molecules of oxygen are transported by a molecule of hemoglobin.

Hb + 4O₂ → Hb4O₂

As blood is transported through the systemic capillaries hemoglobin loses its oxygen to the tissues and becomes hemoglobin again. Hemoglobin shows progressive increase in the affinity for O₂ after the first two molecules of O₂ are taken up by the heme.

The oxygen carrying capacity of the hemoglobin is dependent on the pigment it contains which in turn depends on the iron content for oxygen combining capacity.

The amount of iron present in the blood is minute about 0.334% of the hemoglobin molecule or 0.04 to 0.05% of the blood itself. Each gram of Hb combines with a maximum of 1.34 ml of oxygen.

In the lung (PO₂ 100 mm Hg) the oxygen binds with Hb which shows 97% saturation. One hundred ml of blood containing approximately 15 grams of Hb, can carry approximately 19.4 ml of oxygen.

In the tissue capillaries, (PO₂ 40 mm Hg) Hb is 72% saturated and contains 14.4 ml of oxygen per 100 ml of blood, which indicates oxygen release from the Hb into the tissues. Thus under normal resting conditions about 5 ml of oxygen is transported by each 100 ml of blood during each cycle to the tissues. During heavy exercise this is increased to about 15 times normal. The oxygen hemoglobin dissociation curve is “S” shaped or sigmoid shaped.

Myoglobin (Muscle hemoglobin)

Myoglobin (Muscle hemoglobin) is a true hemoglobin and functions to store oxygen in the muscle. It contains only one heme and a polypeptide chain. It contains only one iron atom and can therefore store only one molecule of O₂.

Its molecular weight is approximately 17,000, which is four times less than Hb. Hence it can pass through glomerulus.

The oxygen dissociation curve with myoglobin is hyperbolic (very steep). The appearance of myoglobin in the urine is referred to as myoglobinuria or azoturia, which is a very characteristic symptom of Monday Morning Sickness in Horses.

Carboxyhemoglobin (HbCO)

Haemoglobin has 200 times more affinity for carbon monoxide than oxygen.

Hb+ CO → HbCO

Carbon monoxide firmly attaches with the Fe⁺⁺ molecules of heme, thus interferes with the transport of O₂ as oxy Hb. 0.1% of CO in inspired air will convert 20% of Hb into HbCO within 30 to 60 minutes.

Oxygen under higher partial pressure is the only means to reverse the reaction.

HbCO + ↑PO₂ → HbO₂ + CO

Methemoglobin (ferrihemoglobin)

Methemoglobin (ferrihemoglobin) is formed by the oxidation of ferrous iron to ferric iron. During the circulation of blood about 1% of methemoglobin is formed by the oxidation of ferrous iron to ferric iron. Ferrihemoglobin cannot combine with oxygen, hence is useless as a respiratory pigment in the blood. It produces dark colored blood. Glutathione (GSH) in the erythrocytes prevents the excessive oxidation of ferrous iron into ferric iron.

Chemicals like nitrates, sulphanamides, aminophenol and acetanilide cause increased concentration of methemoglobin in the blood. Horse blood at normal conditions shows significant amounts of methemoglobin. The normal blood of dog and cat has about 1% of methemoglobin.