CHAPTER 3
Lipids: Steroid Metabolism
JAMES K. GRANT*
Biochemistry Department,
Edinburgh
University,
Scotland
I. The Biosynthesis of Steroids
163
A. Sterol Biosynthesis
163
B. Steroid Hormone Biosynthesis
172
C. Biosynthesis and Transformation of the Bile Acids
184
II. The Metabolism of Steroids
189
A. Introduction
189
B. Metabolism of Sterols by Insects
189
C. Microbiological Transformation of Steroids
190
D. Absorption and Metabolism of Sterols by Animals
191
E. Metabolism of Steroid Hormones
192
F. Conclusions
197
References
198
I. The Biosynthesis of Steroids
A. STEROL BIOSYNTHESIS
1. Introduction
For some time it was believed that plants alone could synthesize
sterols, and that the C 27 compound cholesterol, typical and principal
sterol of animal cells, was derived from plant sterols in the diet. This
belief became untenable when it was observed in balance studies that
animals could excrete more cholesterol than could be accounted for by
ingested sterols, and that the rabbit cannot convert dietary plant sterols
to its characteristic tissue sterol, cholesterol (I).
It now appears that insects require sterols in their diet (2); yeasts,
molds, higher plants and mammals, and birds can synthesize sterols.
Cholesterol occupies a central position in the biogenesis of steroids
insofar as it has been shown to be a precursor of the hormonal steroids
and bile acids of higher animals. Furthermore the biosyntheses of the
more complex C 28 , C 29 , and C 30 sterols of plants and lower organisms
* Present address: Steroid Biochemistry Department, Glasgow University, Scotland.
163