In accordance with an embodiment, described herein is a system and method for managing APIs exposed from proxy services executing in a service bus runtime. Each proxy service can be exposed as an API entry in an API catalog, where the API entry can be annotated with metadata, and published to an API portal. The published API entries can be subscribed to by an API consumer for an application. The application can pass an API key when invoking a proxy service corresponding to a subscribed API entry. A proxy interceptor associated with the corresponding proxy service can intercept the service invocation, and delegate the service invocation to an API managing component. The proxy interceptor can provide the API key and a proxy service reference to the API managing component, which can use the API key to collect usage metrics of the API subscription for display at the API portal.