Albumin is an important factor in the control of serum free fatty acid flux in both male and female mice

Albumin knockout (Alb^-/-) mice exhibit a low plasma free fatty acid (FFA) concentration, but it was not known if the suppressed concentration reflects a lower rate of appearance (Ra) of FFA in the circulation (i.e., lower FFA flux) or if the absence of albumin alters the relationship between FFA flux and concentration. For understanding the role of albumin in FFA transport through the bloodstream, it is not sufficient to rely on FFA concentration data alone. Therefore, we developed a method to study FFA kinetics in Alb^-/- mice. Using an albumin-free formulation of [U-¹³C]palmitate tracer, serum FFA kinetics were tested in Alb^-/- and wildtype (WT) mice. Results indicate that the flux of FFA in serum of Alb^-/- mice was significantly lower than in WT mice (P<0.05), while albumin deficiency did not alter the relationship between FFA flux and concentration. Next, to test if suppressed lipolysis might have also been involved in the suppressed FFA kinetics, gene expression of a lipolytic enzyme (adipose triglyceride lipase, Atgl) and a marker of lipolysis (phosphorylation of hormone sensitive lipase, p-HSL) were measured in adipose tissue. In contrast to the low FFA flux in Alb^-/-, both Atgl gene expression and p-HSL protein were significantly higher in adipose tissue of Alb^-/- than in WT mice (P<0.05). Thus, the low FFA flux in Alb^-/- appeared to be driven by the absence of albumin's FFA binding functions rather than through regulation of lipolysis, indicating that albumin is an important factor in determining the flux of FFA in circulation.