A new version of an old villain gets introduced by Pasko, Swan and Blaisdel in Superman 310 (April 1977).
The criminal organization Skull is back in this story, as Superman stops their raid on STAR Labs. Albert Michaels, the head of that institute, gets saved by Superman, but we see later on that he is actually working with the thieves.
Clark Kent learns that Lois has been considering moving to another city, and she comes over to his apartment to discuss their relationship. She admits that she does not feel that she is being fair to Clark, as he is her second choice, after Superman. Their heart-felt, if bizarre, conversation gets interrupted by Morgan Edge, who calls to tell them about Steve Lombard’s kidnapping. Superman uses his heat vision to lock Lois into the bathroom, where she remains for six hours, to her great displeasure.
Lombard is merely being hung out as bait for Superman, who meets the new Metallo, the brother of the original, transformed by Skull and given a synthetic kryptonite heart. Nice powers, shame about the face.
The synthetic kryptonite hurts Superman, but not so much that it incapacitates him, and he takes down Metallo, who appears to die simply through force of will. In fact, he is not dead at all, but Superman does not realize that.
This new Metallo comes back in a few months, and will remain a significant foe of Superman until Crisis on Infinite Earths.