As others have pointed out, the bottom shelf is not designed to carry the weight of the cabinet on its own.
I would like to offer some alternative solutions.
First is the Rockler Workbench Caster Kit
The adapter plate included in the kit makes them removable. These are heavy duty, designed for shop use. The reason I like this option is that the "island" would only be mobile when you want to move it and it would rest on the floor when in use. This is important because you want a stable work platform that will not move if you lean on it. The down side to these is that they are not attractive for a kitchen/home environment and they stick out quite a bit and would be a trip hazard. that is where the bracket included in the kit come in. The bracket allows you remove the casters when not needed and put them on only when you need to move the island. Only the adapter would remain on the cabinet, not too unattractive and you could paint them to match.
Next are these corner casters. These would be in the void space under the cabinet as you had planned, they would raise the cabinet 1 inch. The downside is they can not be locked into place. And as mentioned by April they would possibly dimple a softer flooring materiel. I would use machine bolts or hex bolts with washers and nuts through the side of cabinet to secure them. The hex head of the bolt would be visible on the side of the panels but you could paint or otherwise disguise them. These casters allow the weight of the cabinet to rest on the side panels so you would not have the weight on the bottom shelf.
I am not a affiliated with Rockler, i just love their products.