Pharmaceuticals Produced by Microorganisms

PHARMACEUTICALS PRODUCED BY MICROORGANISMS

A)  Dextrans

Dextrans are polysaccharides produced by lactic acid bacteria, in particular members of the genus Leuconostoc (e.g. L. dextranicus and L. mesenteroides), following growth on sucrose. These sugar polymers first came to the attention of industrial microbiologists in sugar refineries where large gummy masses of dextran clogged pipelines. Dextran is essentially a glucose polymer consisting of (1–6)-α-links of high but variable molecular weight (15 000–20 000 000; Figure 26.1). Growth of the dextran producing strain is carried out in large fermenters using media with a low nitrogen but high carbohydrate content. The average molecular weight of the dextrans produced will vary with the strain used. This is important, since the laboratory or clinical utility of the dextran is dependent on a defined molecular weight. Two main methods are employed for obtaining dextrans of a suitable molecular weight. The first involves acid hydrolysis of very high molecular weight polymers, while the second uses small preformed dextrans as templates for the polymerization process. These templates are added to the culture fluid to produce dextrans of shorter chain length. Once formed, dextrans of the required molecular weight are obtained by precipitation with organic solvents prior to formulation.

Dextrans are produced commercially for use as plasma substitutes (plasma expanders), which can be administered by intravenous injection to maintain or restore the blood volume, and for application to ulcerated wounds or to burns where they form a hydrophilic layer that absorbs fluid exudates.

A summary of the properties of the different types of dextrans available is presented in Table 26.1. Dextrans for clinical use as plasma expanders require a molecular weight between 40 000 and 300 000. Polymers below 40 000 molecular weight are excreted too rapidly from the kidneys, while those above 300 000 molecular weight are potentially dangerous because of retention in the body. In practice, infusions containing dextrans of average molecular weights of 40 000, 70 000 and 110 000 are commonly encountered.

In the USA, dextran injections with average molecular weights of about 75 000 are also available.

Iron dextran injection, containing a complex of iron hydroxide with dextrans of average molecular weight between 5000 and 7000, is used for the treatment of iron-deficiency anaemia in situations where oral therapy is either ineffective or impractical. The sodium salt of sulphuric acid esters of dextran, i.e. dextran sodium sulphate, has anticoagulant properties comparable with those of heparin and is formulated as an injection for intravenous use.

B)   Vitamins, Amino Acids And Organic Acids

Several chemicals used in medicinal products are produced by fermentation.

i)  Vitamins

Vitamin B₂ (riboflavin), one of the B group of vitamins, is present in milk, liver, kidneys, cereals and vegetables and is also synthesised by intestinal flora in carbohydrate-rich diets. Vitamin B₂ deficiency, although rare, is characterized by symptoms that include an inflamed tongue, dermatitis and injury to the bone marrow. In genuine cases of malnutrition these symptoms will accompany those induced by other vitamin deficiencies. Riboflavin is produced commercially in significant yields by the moulds Eremothecium ashbyii and Ashbya gossypii and some bacteria including B. subtilis (Table 26.2).

Pernicious anaemia is a fatal disease first reported in 1880, but it was not until 1926 that it was discovered that eating raw liver effected a remission. The ‘anti-pernicious’ ingredient was subsequently isolated and called vitamin B₁₂ or cyanocobalamin. Vitamin B₁₂ was initially obtained from liver but during the 1960s it was determined that it could also be obtained as a by-product of microbial metabolism (Table 26.2). Hydroxycobalamin is the form of choice for therapeutic use and can be derived either by chemical transformation of cyanocobalamin or directly as a fermentation product.

Biotin, formerly known as vitamin H, is now regarded as another member of the vitamin B family and is found in similar food types. Biotin acts as an essential cofactor in chemical reactions that maintain normal metabolic function. It is also an essential growth factor for some bacteria. Its chemical structure was established in the early 1940s and a practical, highly stereospecific, chemical synthesis enabled d-biotin, identical to that found in yeasts and other cells, to be produced.

ii)  Amino acids

Amino acids find applications as ingredients of infusion solutions for parenteral nutrition and individually for the treatment of specific conditions. They are obtained either by fermentation processes similar to those used for antibiotics or in cell-free extracts employing enzymes isolated from bacteria (Table 26.2).

iii)  Organic acids

Examples of organic acids (citric, lactic, gluconic) produced by microorganisms, together with pharmaceutical and medical uses, are given in Table 26.2. Citric and lactic acids also have widespread uses in the food and drink and plastics industries. Gluconic acid is also used as a metal-chelating agent in, for example, detergent products.