From Soybean Phytosterols to Steroid Hormones

Abstract

Phytosterols, which are structurally and physiologically similar to cholesterol, are a large group of steroidal triterpenes. They are essential to maintaining normal function in plant cell membranes. In recent years, some of their beneficial effects on human health have come to light. Phytosterols can lower intestinal cholesterol absorption, thereby reducing serum levels of low-density lipoproteins and the risk of atherosclerosis. The pharmaceutical industry has a long history of converting phytosterols to therapeutic steroid hormones by microbial transformation. One commercially available phytosterol product is a phytosterol mixture extracted from soybean oil deodorizer distillate. Soybean phytosterols usually include four sterols: ┚-sitosterol, stigmasterol, campesterol, and brassicasterol, all of which make good raw materials for the production of steroid hormones because of their typical Aring molecular structure with a 3┚-hydroxyl group and a 5,6-double bond (Fig. 1). Two kinds of steroid hormone intermediates can be produced from soybean phytosterols through microbial transformation. The first of these are the C19-steroids, which include androsta-4-ene-3,17-dione (AD), androsta-1,4-dien-3,17-dione (ADD), 9┙-hydroxyandrostra-4-ene-3,17-dione, and testosterone; the second are the C22-steroids, such as 20- carboxy-pregna-l,4-dien-3-one and 20-hydroxymethylpregna-1,4-dien-3-one (Fig. 2). C19- steroids are the products of complete side chain cleavage. They can be used as precursors to almost all kinds of steroid hormones, including sex hormones, anabolic steroids, and even adrenocortical hormones. C22-steroids are the products of truncated side chain. These make good precursors to adrenocortical hormones. The chemical conversion of sapogenins to steroids is a well-established alternative to microbial transformation of phytosterols to steroids. This method has many shortcomings, however, such as higher costs, more steps, low yield, the waste of land resources, and the destruction of wild plant resources. In light of this, microbial conversion of soybean phytosterols to steroids shows great value in the synthesis of steroid hormones. Here, we summarize our knowledge of the occurrence of phytosterols in soybeans and the technology that can be used to transform them from phytosterols to steroids through the regulation and modification of microbial catabolism. Based on analysis of the metabolic mechanisms of phytosterols and the bottlenecks inherent in the microbial transformation process, we will also discuss areas for development and improvement.