NAD(+)-linked isocitrate dehydrogenase in fish tissues.

Area of Science:

• Biochemistry
• Enzymology
• Comparative Physiology

Background:

• NAD(+)-linked isocitrate dehydrogenase (NAD(+)-ICD) is a key enzyme in the tricarboxylic acid (TCA) cycle.
• Understanding the presence and kinetics of NAD(+)-ICD in fish is important for metabolic studies.

Purpose of the Study:

• To investigate the distribution and kinetic properties of NAD(+)-linked isocitrate dehydrogenase in fish.
• To characterize the enzyme's regulatory mechanisms and its role in fish metabolism.

Main Methods:

• Enzyme activity assays were performed on tissue homogenates from trout and eel.
• Partial purification of the enzyme from trout heart was conducted.
• Kinetic parameters were determined under various conditions, including the presence of substrates, cofactors, and inhibitors.

Main Results:

• NAD(+)-ICD was detected in various tissues of trout (brain, heart, gills, kidney, liver, muscle) and eel (liver, muscle).
• Highest enzyme activities were observed in trout brain, with lowest activities in trout and eel white muscle.
• The enzyme's kinetics for DL-isocitrate were sigmoidal without ADP and hyperbolic with ADP, indicating allosteric regulation. Mn(2+) or Mg(2+) were required for activity, with Mn(2+) yielding higher rates. NADH and ATP acted as potent inhibitors.

Conclusions:

• The presence and kinetic properties of NAD(+)-ICD in fish are consistent with its role as a regulatory enzyme in the TCA cycle.
• The enzyme's regulation by ADP, metal ions, and inhibitors suggests a significant role in controlling cellular energy metabolism in fish.