Hexokinase isozyme distribution in human skeletal muscle

The role of hexokinase in glucose metabolism is well established, but the subcellular localization of its isozymes in human skeletal muscle remains unclear. Prior research has shown that hexokinase exists in two main forms, types I and II, and both are present in skeletal muscle. However, the distribution of these isozymes between cytosolic and particulate fractions has not been fully characterized in humans. This gap motivated the current investigation into how these isozymes are localized within muscle cells. Understanding their distribution is important because hexokinase binding to subcellular structures may regulate glucose phosphorylation. Previous studies have focused on hexokinase activity in general but have not specifically examined the isozyme-specific localization in human tissue. This paper addresses that limitation by using a combination of biochemical methods to determine the isozyme distribution. The findings may help clarify how glucose metabolism is regulated at the cellular level in skeletal muscle. No prior work had resolved the isozyme-specific localization in human muscle, making this study a novel contribution.

The primary aim of this study was to determine the subcellular distribution of hexokinase isozymes I and II in human skeletal muscle. The researchers sought to clarify whether these isozymes are predominantly localized in the cytosol or particulate fraction of muscle cells. This question is important because hexokinase binding to mitochondria or other structures may influence glucose metabolism. The study focused on healthy lean individuals to avoid confounding factors like obesity or metabolic disease. The researchers used muscle samples from the vastus lateralis, a major leg muscle, to ensure relevance to human physiology. They aimed to validate a thermal inactivation method for distinguishing isozymes and to quantify their relative contributions to total hexokinase activity. The motivation for this work was to better understand how glucose is phosphorylated in muscle cells under normal conditions. No prior work had resolved the isozyme-specific localization in human muscle, making this study a novel contribution.

The researchers obtained vastus lateralis muscle samples from healthy lean individuals and prepared a homogenate. They separated the homogenate into particulate and cytosolic fractions using centrifugation at 45,000g. To assess hexokinase activity, they used a high-performance liquid chromatography-based assay. Ion-exchange chromatography was also employed to distinguish between isozymes I and II. A modified thermal inactivation method was validated for isozyme separation. The particulate and cytosolic fractions were analyzed separately for hexokinase activity. The study measured both total hexokinase activity and the relative contributions of isozymes I and II. The researchers compared activity levels in the two fractions to determine isozyme localization. These methods allowed them to quantify the distribution of isozymes within human skeletal muscle.

Hexokinase activity was significantly higher in the particulate fraction compared to the cytosolic fraction. The mean hexokinase activity in the particulate fraction was 3.88 ± 0.65 U/g wet weight or 0.64 ± 0.11 U/mU creatine kinase (CrK). In the cytosolic fraction, the activity was 0.45 ± 0.22 U/g wet weight or 0.07 ± 0.03 U/mU CrK. Hexokinase I accounted for 70–75% of total activity, while hexokinase II contributed 25–30%. Nearly all (95%) of hexokinase I activity was found in the particulate fraction. Hexokinase II was also largely (72%) in the particulate fraction, but 28% was in the cytosol. Within the particulate fraction, hexokinase I and II contributed 81% and 19%, respectively. In the cytosolic fraction, their contributions were 37% and 63%. These findings suggest a distinct subcellular localization pattern for the two isozymes.

The study found that hexokinase I is predominantly localized in the particulate fraction of human skeletal muscle, consistent with its known high affinity for mitochondria. Hexokinase II was also largely found in the particulate fraction but had a notable presence in the cytosol. These findings suggest that the two isozymes have distinct subcellular distributions. The authors propose that this distribution may influence glucose phosphorylation in muscle cells. The study validated a thermal inactivation method for distinguishing isozymes, which could be useful in future research. The results provide a clearer picture of how hexokinase activity is regulated in human muscle. The authors suggest that these findings may help explain how glucose metabolism is modulated at the cellular level. No prior work had resolved the isozyme-specific localization in human muscle, making this study a novel contribution.