IV VITAMIN BIOSYNTHESIS IO9 



mediate in these reactions. The enzyme catalyzing the activation of the COj has 

 been separated from the one concerned with the CO2 fixation reaction. The heart 

 enzyme system also effects the breakdown of ^-hydroxy-^i-methylglutaryl-CoA to 

 acetoacetate and acetyl-CoA (Bachhawat et al., 1954). 



Consistent with the possibility that P-methyl-crotonyl-CoA is a carotene precursor is 

 the fact that the alpha carbon of acetate is incorporated into the lateral methyl groups 

 of carotene; on the other hand, the vicinal quaternary carbon atoms of the aliphatic chain 

 are derived from the carboxyl carbon of acetate (Grob and Butler, 1954). The synthesis 

 of carotene has been studied in the mold, Phycomyces blakesleeanus. In the presence of p-ionone, 

 both leucine and asparagine stimulate carotene synthesis. Although leucine- i-''*C is not 

 incorporated into the carotene of this organism, leucine-2-'''C is incorporated. This is 

 consistent with the fact that the carboxyl group of leucine is lost during the conversion 

 of the amino acid to [3-methylcrotonyl-CoA (Goodwin and Lijinsky, 1951; Chichester 

 etal., 1955). 



A condensation cf two "isoprene-like" molecules would give rise to a 10 carbon 

 compound having an isoprene chain. A 20 carbon compound could be formed 

 by the condensation of two 10 carbon compounds or by the condensation of a 5 

 carbon compound with a 15 carbon compound. In animal tissues, (^-carotene is 

 cleaved to two molecules of vitamin A. 



Evidence exists which indicates that the 30 carbon trihydroterpene, squalene, 

 an intermediate in the synthesis of cholesterol and other steroids, is also probably 

 formed by a condensation of six "isoprene" units. Rat liver extracts which in- 

 corporate acetate- 1 -^"^C into [3-methylcrotonate also form labelled cholesterol. The 

 conversion of acetoacetate-^'*C to cholesterol takes place without the preliminary 

 formation of two carbon compounds (Brady and Gurin, 1 95 1 ) . The incorporation of 

 i-'^^C-acetyl-CoA into liver cholesterol is lowered to less than half by the addition of 

 ^-methylcrotonate or p-hydroxy-^-methylglutarate (Rabinowitz and Gurin, 1 953) ' . 



Squalene occurs naturally in shark liver oil, yeast, human dermatoid cysts, and has been 

 isolated in trace amounts from rat liver and intestine (Langdon and Bloch, 1953a). Bio- 

 synthetically labelled squalene is efficiently converted to cholesterol in mouse tissue in vivo 

 (Langdon and Bloch, 1953b). The observed distribution of the methyl and carboxyl 

 carbons of isotopically labelled acetate in tissue cholesterol is also consistent with the role 

 of squalene as a sterol precursor (Langdon, 1954; Fig. 49). It will be noted that the hypoth- 

 esis predicts that the two central carbons of squalene shall be derived from the carboxyl 

 carbon of acetate. In accord with this hypothesis, carbons 11 and 12 of the C ring of 

 ergosterol (which correspond to the central carbon atoms of the squalene) of yeast grown 

 in the presence of acetate- i-'^C were equally labelled (Langdon and Bloch, 1953a; Dauben 

 and Hutton, 1956). Both the carboxyl and methyl groups of acetate are converted to 

 squalene in the rat. 



Acetate- '■'C is also converted to the CjQ-steroids, lanosterol and agnosterol by rat liver 

 homogenates (Clayton and Bloch, 1956a). The biosynthetically labelled lanosterol is in turn 

 converted to cholesterol by the same enzyme system (Clayton and Bloch, 1956b). Both 

 lanosterol and agnosterol occur in trace amounts in liver. In experiments with yeast, 

 acetate- '''C gave rise to labelled ergosterol, squalene and zymosterol. When labelled 

 zymosterol was administered to rats, cholesterol became labelled (Schwenk et al., 1955). 

 It could be formed from lanosterol by the loss of three methyl groups. It will be noted 

 that the double bonds of zymosterol and lanosterol are in corresponding positions in the 

 two molecules (Fig. 49). The cyclization of squalene to lanosterol by rat liver enzymes 



^ See Addendum, Note 5, p. 123. 



Literature p. 124 



