Our measurements of ACC1 and ACC2 transcript levels in rodent samples agree with published data 
showing high levels of ACC1 in adipose, high levels of ACC2 in heart and skeletal muscle tissues, and higher levels of ACC1 than ACC2 in liver. Also in agreement with published data 
, our microarray profiling of mouse brown adipose shows higher ACC2 expression than in mouse white adipose (data not shown). Human tissues show a very different expression pattern (). Human adipose expresses ACC2 at levels four times higher than ACC1, a higher level than observed in human skeletal muscle. Human liver expresses more ACC2 than ACC1. Although ACC2 was first cloned from human adipose tissue 
, attention has focused on the role of ACC2 in fatty-acid oxidation in muscle tissue, and, to our knowledge, the high expression of ACC2 in human adipose has not previously been reported. Furthermore, microarray expression profiling in 53 dog and 65 rhesus monkey tissues (data not shown) show highest ACC2 levels in dog and rhesus adipose. Thus, ACC2 expression in primate and canine adipose is different than in rodent adipose, and may represent a species difference in fatty-acid regulation.
Although we have shown there are significant differences in ACC2 transcript and protein expression between rodent and human tissues, we cannot rule out the possibility that a part of the observed differences is due to variation in tissue dissection, RNA isolation, or sample pooling. However, adipose is a relatively homogeneous tissue, such that different dissection methods would not be expected to produce large changes in the cell-type composition of the isolated adipose tissue. Secondly, adipose samples collected from multiple vendors, for both individuals and pools of individuals, consistently show similar mRNA expression profiles that cluster together in a background of hundreds of other tissue samples (data not shown), suggesting that dissection and RNA isolation differences tend to produce relatively small effects on mRNA profiles.
We demonstrate the existence of a novel ACC2 isoform (ACC2.v2) that is expressed in human white adipose tissue and has enzymatic and DNL activity. The protein is smaller (~255 kDa) than that of ACC1 (~265 kDa) and the known ACC2 protein (ACC2.v1, ~277 kDa) (, Supplemental Figures S1
). This form lacks the hydrophobic N-terminus found in the known ACC2 isoform, including the putative mitochondrial localization sequence but has all three catalytic domains essential for ACC enzyme activity. The ACC2.v2 mRNA accounts for the majority of ACC2 transcripts in testis, over 20% in adipose and approximately 5% in skeletal muscle. ACC2.v2 is also found in mouse and rat and is expressed at ratios similar to ACC2.v1 in these species (), suggesting that the novel isoform is similarly regulated between species.
In human adipose, over 20% of the highly expressed ACC2 mRNA is in the form of ACC2.v2, which is expressed at levels comparable to ACC1. ACC2.v2 and ACC1 transcripts encode proteins that are 76% identical (87% similar), and the biotin carboxylation and carboxyl transferase regions are 84% and 81% identical (93% and 91% similar), respectively. The lack of the first coding exon of ACC2.v1 makes the ACC2.v2 protein sequence more similar to ACC1. Previous enzyme kinetics show only lower Km
for ACC1 than for ACC2 for acetyl-CoA and citrate 
, and our data suggest ACC2.v1 and ACC2.v2 have comparable ACC activity. All recombinant ACC isoforms tested, ACC1, ACC2.v1, and ACC2.v2, showed DNL activity in cells. However, given the moderate DNL activity of ACC2.v2 despite its level of protein expression, ACC2.v2 may not be as efficiently coupled to DNL pathway as ACC1. While cellular activity is also dependent on mRNA translational rates and phosphorylation states 
, these expression and enzymatic and DNL activity results suggest that ACC2 isoforms, both ACC2.v1 and ACC2.v2, are capable of contributing to fatty-acid synthesis in human adipose tissue. This hypothesis would also help explain the observation that liver-specific ACC1 deficient mice showed hepatic DNL 
, and is consistent with the observation by Oh et al. 
mice showed not only increased fatty-acid oxidation but also decreased fat levels in adipose. Finally, our results also suggest ACC2 specific inhibition could result in different physiological effects in humans than in rodents.