Nonequivalence of the two subunits of horse erythrocyte glutathione transferase in their reaction with sulfhydryl reagents.

The research article revolves around studying the enzyme glutathione transferase from horse erythrocytes, which comprises of two subunits that react differently when exposed to certain chemicals, suggesting an asymmetrical association or heterodimeric structure of these subunits.

Glutathione Transferase Purification and Analysis

• The researchers began their work by purifying glutathione transferase, an enzyme extracted from horse erythrocytes, for further investigation.
• They examined some molecular and kinetic properties of this enzyme to get insights into its behavior and characteristics.
• The enzyme appeared to be a ‘dimeric’ protein, essentially meaning it consists of two identical subunits, each having a weight around 23 kDa.
• These subunits were found to behave the same way in both sodium dodecyl sulfate and urea electrophoresis.

Research Tools and Findings

• Various research techniques, including studying amino acid composition, substrate specificities and sensitivity to inhibitors, CD spectra, and immunological studies, were applied to learn more about the enzyme.
• Through these analyses, the researchers discovered evidence suggesting that this enzyme belongs to the pi class transferases.
• Each dimeric form of the enzyme had only two reactive thiol groups. It became clear that the integrity of these thiol groups was critical to the enzyme’s activity.

Distinctive Behavior of Thiol Groups

• Interestingly, these thiol groups reacted in diverse ways with different sulfhydryl reagents, such as iodacetamide, bromopyruvate, N-ethylmaleimide, and 1-chloro-2,4-dinitrobenzene.
• Many disulfides also interacted with one thiol group much quicker than the other – the reaction rate differed by 5 to 10 times.
• When these mixed disulfides went through reactivation by dithiothreitol, the two different rates of reactivation were observed, further underlining the distinctive behavior of the two thiol groups.

Non-Identical Subunits or Heterodimeric Substantial Structure?

• The different reactivity of the thiol groups and all other findings mentioned in the paper led the authors to propose either a nonsymmetrical association of two identical subunits or a heterodimeric protein structure where subunits, despite similar charge and size, might not be identical.
• This paper showcases that even though the subunits seemed identical initially, their different reactions to chemical reagents suggest a more complex structure or relationship.