Nicotinamide nucleotide synthesis in regenerating rat liver

Area of Science:

• Biochemistry
• Cell Biology
• Physiology

Background:

• Nicotinamide nucleotides, including NAD and NADP, are crucial coenzymes involved in numerous metabolic processes.
• Liver regeneration following partial hepatectomy is a complex process involving rapid cell division and altered metabolic demands.

Purpose of the Study:

• To investigate the changes in nicotinamide nucleotide concentrations and content in rat liver during the process of regeneration after partial hepatectomy.
• To correlate these changes with other key events in liver regeneration and to understand the regulation of NAD synthesis in rapidly dividing tissues.

Main Methods:

• Measurement of nicotinamide nucleotide (NAD and NADP) concentrations and total content in rat liver at various time points after partial hepatectomy and sham hepatectomy.
• Administration of nicotinamide to assess its effect on liver DNA and NAD levels, and to study the time course of NAD synthesis capacity.
• Assay of liver enzyme activities, specifically NAD pyrophosphorylase (EC 2.7.7.1) and NMN pyrophosphorylase (EC 2.4.2.12).

Main Results:

• Partial hepatectomy led to a significant decrease in NAD content and concentration by 24 hours, while NADP levels were also depressed after both partial hepatectomy and sham operations.
• Nicotinamide injections showed an inverse correlation between NAD and DNA content post-hepatectomy, and the liver's ability to synthesize NAD from nicotinamide was impaired early after surgery.
• NAD pyrophosphorylase activity increased significantly after 12 hours, while NMN pyrophosphorylase activity remained low throughout the observation period.

Conclusions:

• The regulation of NAD synthesis in rapidly dividing liver cells is influenced by the availability of precursors and cofactors, rather than solely by maximum enzyme activities.
• Competition between NAD synthesis and nucleic acid synthesis for common precursors and cofactors likely contributes to the observed low concentrations of nicotinamide nucleotides in regenerating liver tissue.