acrylic on enamel, yes? no?

Everything that Scott, Tom, and Darren have said is correct—but are we assuming that your enamel is a matte finish? If it's a gloss, I'd sand it with very fine grit (~1500-2000) and wash it with a surfactant  cleaner (Windex, Simple Green, etc.) and rinse thoroughly with water.

Gloss surfaces are intended not to have things stick them, and some use silicone to enhance the gloss. Even some enamels bead up or crawl on silicone finishes.

I have painted many times acrylics over an enamel base and as Tom said as long as the enamel is cured (allowed to dry properly 24-48 hours) you should have no worries at all.

Also make sure the enamel is cured first or you might end up with cracking or bubbling of the paint.

Yes, acrylic adheres very well to a base of enamel.  In fact it will generally adhere much better to an enamel (or laquer) base than it will to bare plastic.