Translation not working for Constant strings in Plugin

I want to translate constant strings in my plugin defined as:
define( 'CONSTANT', __( 'string-A', 'textdomain' ) );.
The 'string-A' is available for translation and I have uploaded the mo files in the desired language including this string but in the WordPress it still reflects as English string. Translation is working for all other strings except the one defined as constants in my config file. Any idea why is this happening?