Kinetics of myocardial phospholipase D

Area of Science:

• Cardiovascular Biology
• Enzymology
• Cellular Biochemistry

Background:

• Myocardial phospholipase D (PLD) is present in key subcellular membranes like sarcolemma (SL) and sarcoplasmic reticulum (SR).
• Understanding the kinetic differences between PLD in these distinct membrane locations is crucial for elucidating its specific functions.

Purpose of the Study:

• To kinetically characterize phospholipase D (PLD) activity in isolated rat heart sarcolemma (SL) and sarcoplasmic reticulum (SR) fractions.
• To compare the hydrolytic and transphosphatidylation activities of SL PLD versus SR PLD.

Main Methods:

• Isolation of sarcolemma (SL) and sarcoplasmic reticulum (SR) fractions from rat heart tissue.
• Enzymatic assays measuring the formation of phosphatidic acid (hydrolytic activity) and phosphatidylethanol (transphosphatidylation activity) to determine kinetic parameters (Vmax and Km).

Main Results:

• Sarcolemma phospholipase D (SL PLD) exhibited significantly higher Vmax values for both hydrolytic (373 vs. 70 nmol/mg protein/h) and transphosphatidylation (415 vs. 60 nmol/mg protein/h) activities compared to sarcoplasmic reticulum PLD (SR PLD).
• SL PLD demonstrated a lower Km for hydrolytic activity (0.46 vs. 0.65 mM) but a higher Km for transphosphatidylation activity (225 vs. 179 mM) relative to SR PLD.
• Distinct kinetic parameters (Vmax and Km) were observed for SL PLD and SR PLD.

Conclusions:

• The differing kinetic profiles of SL PLD and SR PLD suggest they may be distinct isoforms or localized to different membrane domains.
• These kinetic differences imply that SL- and SR-localized PLD activities are likely regulated independently.
• Independent regulation suggests unique roles for sarcolemma and sarcoplasmic reticulum PLD in normal cardiac physiology and pathological conditions.