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Another important product obtainable from vegetable oil wastes is sterol. Although steroid drugs represent only a small part of the world market of pharmaceuticals, there is a great demand for new and cheaper steroid raw materials for their production (Dias et al., 2002). The choice of starting material has always had a critical impact on the steroid-manufacturing industries. The most common and economical process for the production of steroid pharmaceuticals is the partial synthesis from relatively inexpensive steroid raw materials of animal and plant origins. Diosgenin and other sapogenins were the preferred starting materials until the 1970s. Soybean sterols obtained from soybean oil processing were plentiful and cheap, and included a large fraction of stigmasterol which can be easily converted to progesterone, thus being an excellent alternative to diosgenin (Dias et al.,

2002). Stigmasterol represented about 15% of the total precursors used in the USA, becoming, with diosgenin, one of the main raw materials for industries producing steroids of the pregnane, anderostane and estrane series. Whereas stigmasterol could be degraded chemically starting with the oxidative cleavage of the 24-double bond, preserving the steroid ring structure, sterols like sitosterol and cholesterol, with saturated side-chains resistant to selective degradation, were considered low-value or even waste products. However, since the isolation of the first mutant Mycobacterium sp. strain capable of degrading the side-chain of sterols giving 17-keto-ster-oids (Dias et al., 2002) and the development of methods for the chemical addition of the corticoid side-chain to these 17-intermediates, P-sitosterol, the most ubiquitous plant sterol, became a major raw material for the synthesis of corticosteroids, which represents the bulk of the steroid industry. As an alternative to purified sitosterol, mixed sterol concentrates obtained from natural sources such as soya or rape seed, or from industrial wastes (sugarcane and paper industries), have been tested as substrates for the microbial production of 17-ketosteroids (Dias et al., 2002). Over 1000 tonnes of the latter chemicals are produced per year. Additionally, sito-sterol can be converted by other microbial mutants to 9a-hydroxy-17-keto-steroids or 20-carboxy pregnane derivatives (Dias et al., 2002) which are more suitable for corticosteroid synthesis.

Dias et al. (2002) isolated a biodegradable sterol-rich fraction from industrial wastes. Several industrial waste materials were screened for their sterol content. The authors studied the possibility of using these industrial by-products as sterol sources for the microbiological production of 4-androsten-3,17-dione (AD) and 1,4-androsta-diene-3,17-dione (ADD). Two methods of obtaining the sterol fraction from wastes were developed. Sterol-rich (96-98%) fractions were isolated in a yield above 70%, from a tall-oil effluent of the paper pulp industry and from deodorized edible oil. These fractions were subsequently used as a substrate for microbial degradation by a Mycobacterium sp. strain and proved to be easily converted to AD and ADD.

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