Filter my args by the post 1st letter

Hi there,
I filter my WP_Query by this script:

$args = array (
					'posts_per_page' => $posts_per_page,
					'post_type' => $post_type,
					'meta_key' => 'post_views_count',
					'orderby' => 'meta_value_num',
					'showposts' => 160,
					'order' => 'DESC',
					'paged' => $paged,
    				'tax_query' => array(
        				array(
            				'taxonomy' => $term->taxonomy,
            				'field' => 'slug',
            				'terms' => $term->name)
    					));

And I would like to add the filter SUBSTRING(post_title, 1,1) ='z'.
it possiable to implement it?
what’s the best practices to loop only post which start with X letter?
Thanks!