Aging-dependent changes in rat heart mitochondrial glutaredoxins--Implications for redox regulation

Area of Science:

• Mitochondrial biochemistry
• Cardiovascular aging
• Oxidative stress

Background:

• Elderly hearts are more prone to ischemia/reperfusion injury than young hearts.
• Aging reduces cytosolic glutaredoxin 1 (Grx1) in cardiomyocytes, decreasing anti-apoptotic protein expression.
• Glutaredoxin 1 (Grx1) is found in the cytosol and mitochondrial intermembrane space (IMS), while glutaredoxin 2 (Grx2) is in the mitochondrial matrix.

Purpose of the Study:

• To investigate the changes in Grx1 and Grx2 levels and activity in heart mitochondria during aging.
• To determine the activation status of Grx1 and Grx2 in the hearts of elderly rats.
• To explore the factors influencing Grx2 activity in aging hearts.

Main Methods:

• Quantitative analysis of Grx1 and Grx2 levels in subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria from elderly rat hearts.
• Determination of in situ enzyme activity for Grx1 and Grx2.
• Assessment of Grx2 activation using chemical reagents (sodium dithionite) and superoxide-generating systems (xanthine oxidase, rotenone).

Main Results:

• Grx1 decreased by 50-60% in the IMS, while Grx2 increased by 1.4-2.6 fold in the mitochondrial matrix of elderly rat hearts.
• Grx1 was active in the IMS, but Grx2 was predominantly inactive in the matrix, forming inactive dimers with iron-sulfur clusters.
• Inactive Grx2 could be reactivated by sodium dithionite or exogenous superoxide but not by rotenone, suggesting localized insufficient reactive oxygen species (ROS) accumulation.

Conclusions:

• Aging alters the balance of Grx isozymes in heart mitochondria, with decreased active Grx1 and increased inactive Grx2.
• The inactive dimeric form of Grx2, stabilized by iron-sulfur clusters, may contribute to mitochondrial dysfunction in aging hearts.
• Specific conditions of ROS generation are required for Grx2 activation, indicating complex regulation in situ.