Biosynthetic Origin of Individual Sterols

Cholesterol and its 24-methyl and 24-ethyl derivatives do not appear to be inter-converted in higher plants. These and other of the dominant sterols arise by partly separate pathways from cycloartenol. While understood in principle (33, 67, 71), the pathways still require further investigation in terms of precise sequences and the factors that control them (33, 65, 6, 71). 

The 24)5-alkyl route is summarized in Fig. 8.2.5. It shoud be remembered, however, that there remain a number of uncertainties about the sequencing of 

A problem similar to the one with the A -bond exists with a number of other differences in plant sterols. Thus, we have the A /A ratio to explain as well as the 24a-methyl/24)8-methyl and 24-methyl/24-ethyl ratios not to mention the curious fact that 24-ethylsterols of main line plants are always of the a-configura-tion although the 24-methyl analogs are not. In view of the statistics of occurrence, such as that sitosterol is usually the major sterol in main line plants, it would appear that the ratios are rather tightly controlled. This tight control probably constitutes a fine tuning of the function(s) that the sterol pool plays. 

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