Figure 6.

Predicted and measured relative abundance of mass isotopomers of C18:1 methyl ester, after growth on [U-¹³C₆]Glc and unlabeled Suc. From the fragment C18:1_(1-2), representing the terminal biosynthetic acetate unit, the distribution of mass isotopomers was predicted. The polymerization of nine acetate units to a C18 fatty acid molecule can be described by a binominal distribution (compare with Lee et al., 1992): (p + (q₁ + q₂))⁹. q₁, Fraction of (¹³C₁) acetate; q₂, fraction of (¹³C₂)-acetate; and p = 1 − q_{1 −} q₂, the fraction of (¹²C₂) acetate. The expansion of this binominal distribution leads to the relative abundance of mass isotopomers (m₀, m₁, m₂… m_2n) of the fatty acid (Lee et al., 1992). After labeling with [U-¹³C₆]Glc (diluted 20:80 with Suc, as related to hexose units), the fractional labeling in C18:1_(1-2) was determined to q₁ = 0.084, q₂ = 0.267, and p = 0.649. This equals an average ¹³C content of 31%. The predicted distribution (white bars) and the measured molecular ion of C18:1 (black bars) are similar, indicating that the majority of fatty acid molecules was formed from acetate units labeled as measured in the fragment C18:1_(1-2). However, a 4% fraction of fatty acid molecules was much higher labeled. Thus, during fatty acid biosynthesis, a small part of fatty acid molecules has been made mainly from labeled Glc, whereas the bulk of fatty acids was made by a homogeneous mixture of about 30% labeled and 60% unlabeled hexose units.