Lamar Odom -- Best Friend Jamie Dead ... Dies of Apparent Heroin OD

Lamar Odom's best friend, Jamie Sangouthai -- the man Khloe and others claimed fueled Lamar's disastrous crack binge 2 years ago -- has died, and we're told authorities believe it was a heroin OD.

Law enforcement sources tell TMZ, Jamie died Sunday in a hospital in downtown L.A. He was taken to the hospital after someone called 911.

Jamie was featured on Khloe and Lamar's reality show and they highlighted Jamie's problems on the program.

You may recall a crazy video of an obviously high Lamar and Jamie rapping about Khloe.

Khloe just tweeted, "When does it stop? RIP Jamie Sangouthai."

Jamie was 37 years old.