So I'm just curious, is it possible to hit level fifty on one playthrough?

if you mess around in friend's games sure, but just by yourself doing every mission in playthrough 1 with no dlc you'll be like, 38

yes, just sit there and kill scags forever.

Right on, thanks man!

Well, technically yes since enemies can respawn in a lot of areas but it'll take a while. If you play normally by yourself you hit around level 38 or so.

Possible, yes.  Logical, no. 
 
onimonkii is right on the money.  I was Level 38 after beating the game the first time, if I remember correctly. 
 
You might as well just start a Playthrough 2 and level-up faster.  By the time you're like 3/4 the way through you could hit 50. 

It's faster to hit 50 if you just run through your second play through

 Onimonkil - Gave the more proper answer. While technically true that you can kill scags forever I really would not recommend that. It would take way longer then starting up play-through 2, playing in others games online or via DLC and would be crazy boring. 

Thanks for the words everyone. I was only curious because I'm not extremely crazy about starting a second playthrough but... whatever, no big deal you know.