384 VIII. CHOLESTEROL AND RELATED STEROLS 



d. Isovaleric Acid. This acid has proved to be an efficient carbon source 

 for cholesterol, as determined by studies with (Ci^H3)2:CH2-C'400Hi" 

 and C^Mabeled isovaleric acid ((CH3)2:C'^H-CH2-COOH).203 When the 

 doubly labeled isov^alerate was tested by Zabin and Bloch/^^ it was found 

 that the isopropyl carbons (C4 and C4') of the molecule gave rise to a 

 higher isotope concentration than did the carboxyl carbon. This result is 

 interpreted as e\'idence that isovalerate enters the synthetic process with- 

 out alteration of the carbon chain. In a later report, Bloch et a/.'^"^ stated 

 that the conversion efficiency of dimethyl acrylate ((CH3)2:CH-CH:CH-- 

 COOH) is similar to that of methyl isovalerate in cholesterol biogenesis. 

 This suggests that ,5,;S-dimethyl acrylate (senecioic acid) is a step in the 

 metabolism of isovalerate. A similar hypothesis has been proposed by 

 Bachhawat and co-workers. ^"^ It is as yet uncertain whether or not this 

 desaturation is an obligatory step in the oxidation of isovalerate. Until ex- 

 periments have been completed with carboxyl-labeled dimethyl acrylate, 

 no conclusions can be reached. 



The theory that branched-chain acids are intermediates in the biosyn- 

 thesis of cholesterol from acetate has gained support by the demonstration 

 that hydroxymethylglutarate (HOOC • CH2 • COH (CH3) • CH2 • COO- 

 jj)^ 196.208 ajj(j /SjiS-dimethylacrylate-"^'^"^ are synthesized from acetate by 

 rat liver homogenates. The distribution of acetate carbon noted by Rud- 

 ney-"^ in the two acids is identical with that predicated by Wiiersch et 

 aU'^ for the isoprenoid precursor of cholesterol, which Bloch^- considers 

 may arise by way of squalene. Bloch and associates^"* postulated the 

 following possible method by which acetate is used in the synthesis of iso- 

 prenoid units: 



C— COOH 



Cv 



3CH3COOH > C— C > )C=C- 



-i ."> 



C— COOH 



One Possible Method for the Formation of Isoprenoid Units, from Acetate, Which Are 

 Used in the Synthesis of Squalene and Cholesterol'"'^ 



If the above theory is correct, then three molecules of acetate give rise 

 to one isoprenoid unit by way of a six-carbon dicarboxylic acid. Six such 



203 1. Zabin,and K. Bloch, /. Biol. Chem., 192, 267-273 (1951). 



20" K. Bloch, L. C. Clark, and I. Hararv, /. Biol. Chem., 211, 687-699 (1954). 



205 B. K. Bachhawat, W. G. Robinson, and M. J. Coon, /. Am. Chem. Soc, 76, 3008- 

 3099(1954). 



206 H. Rudnev, ./. Am. Chem. Soc, 76, 2595-2596 (1954). 



207 J. L. Rabinovvitz, J. Am. Chem. Soc, 76, 3037-3038 (1954). 



208 J. Wuersch, R. L. Huang, and K. Bloch, J. Biol. Chem., 195, 439-446 (1952). 



